Kajiura Rika is a character in the light novel The Testament of Sister New Devil and its derivative works, created by Uesu Tsuzuri with illustrations by Okuma Suke, and voiced by Yū Shimamura. She is 16 years old, a second-year student at Seisaka Academy, and serves as the vice president of the student council, known for her strict and earnest personality. As a key supporting character in the story, Kajiura Rika shares a close bond with the protagonist Tōjō Basara and maintains friendships with characters such as Nonaka Yuzuki and Tachibana Nanao. Through the contrast between her serious exterior and her underlying emotions, the character is portrayed as a three-dimensional figure. She plays a pivotal connecting role in both the story's daily scenes and its main plotline, with her motivations consistently revolving around her deep value for friendship and adherence to her personal principles.

Noel is a demon character who works as a maid in the anime series [The Testament of Sister New Devil BURST]. Noel is characterized as a classic "Genki Girl" with an exuberant, boisterous, and highly energetic personality. Despite her position as a servant, she brings a lively and distinct presence to her scenes. Noel has a loud, over-the-top, and theatrical way of expressing herself. Her high spirits make her stand out among the more reserved characters in the Moderate Demon Faction. Although she is officially employed as one of Klaus's maids, Noel isn't the most disciplined worker. She frequently skips out on her duties and slacks off whenever she gets the chance. When Basara Toujou and his companions visit the Demon Realm, Noel acts as an accommodating host. She looks after their needs and makes sure they are well taken care of during their stay. Having grown up in an orphanage alongside Yahiro Takigawa (Lars), she shares a deep, lifelong bond with him and remains a good-natured, decent person at heart.

Kaoru Nonaka is a secondary character who is the mother of Yuki Nonaka and Kurumi Nonaka, two of the primary heroines in the story. Because Kaoru Nonaka is a very minor background character in The Testament of Sister New Devil, her specific psychological traits and personality are not deeply explored in the anime or light novel series. Instead, her character is defined by her role as a traditional mother and a strict member of the Hero Clan village.

Raphaeline is a Goddess and a member of the Ten Gods. She is also one of the two mothers of Basara, since his biological mother, Sapphire couldn't return to the Demon Realm with the child of a hero after the Great War so he was transferred into her womb to be born in the Divine Realm. As a result of giving birth to the child of a hero and demon, Raphaeline was punished and imprisoned. Among the gods who held hostility towards the demons, she was among the peaceful ones who desire there to be peace among them so that no more conflict would arise. Raphaeline has also shown to be trusting to the heroes, as she granted Jin Toujou the holy sword, Balmung, to assist him in his fight in the Great War. She was favorable towards Wilbert who called back his troops to the Demon Realm to create a truce and tried to stop the battle that continued between Jin and Sapphire. Though despite her being a peaceful god, Raphaeline could also be a bit childish and emotional as she fought back against both Jin and Sapphire who both got so heated in their fight that they attacked her separately turning their fight into a three-way battle. Raphaeline has also shown to have considerable love for Jin and Sapphire as well as Basara when she sacrificed herself in order to give birth to him and even named him herself despite knowing about the absolute rules she must follow, especially then due to her status.

Sapphire is the younger sister of the previous Demon Lord, Wilbert thus making her the aunt of Mio and Maria Naruse, as well as the biological mother of Basara Toujou. Though not much about her has been revealed, Sapphire is shown to be both compassionate and courageous as she sent back her allies to fight against Jin during the Great War so that no one would get caught up in their battle. In their several battles, she may have developed a hint of a rivalry with Jin since she hated the fact that Raphaeline intervened in their battle despite mentioning that the Great War ended. She is also shown to be cooperative as she assisted Raphaeline, who was meant to be the demon's greatest enemies to rescue Jin from the dimension they were trapped in. Sapphire soon grew to care deeply about Jin and Raphaeline to the point where the three decided to have a child that was theirs. However, after Raphaeline couldn't return from the Divine Realm, she was unable to enjoy a life with Jin and Basara without her. Though having heard about the incident in the Hero Clan, Sapphire met Basara and consoled him about the incident before erasing his memories of him meeting her. She cares enough for Raphaeline is further shown as she went into the Divine Realm in order to save her from even the Ten Gods who imprisoned her.

Ashe is a human character and the mother of Mio Naruse in the anime and light novel series The Testament of Sister New Devil (Shinmai Maou no Testament). Because Ashe is a posthumous character who passed away before the main events of The Testament of Sister New Devil, her personality is never deeply explored or shown in active dialogue. However, the light novels and character backstories provide implicit details about her temperament. Despite being married to Wilbert—the most powerful and feared Demon Lord—she chose a quiet life. She spent her married days confined to a tower for her own safety (due to the anti-human sentiment among conservative demons), finding solace and joy simply in tending to plants. Knowing that giving birth to a demon-human hybrid would put immense strain on her body, she still chose to carry Mio to term, ultimately dying shortly after childbirth. Characters who knew her, such as Ortega, explicitly state that Mio is a spitting image of Ashe. This implies that Ashe likely shared Mio's core traits: a kind, deeply caring, and resilient nature hidden beneath a vulnerable exterior.

Shella is the mother of Maria Naruse and Rukia. Known for her cute, almost child-like appearance, Shella was once the most powerful succubus. She was captured as a hostage by Zolgear but later rescued by Yahiro Takigawa. Even after her rescue, Shella continues to pop up and surprises everyone with her playful and open personality, often finding herself in unexpected and funny situations. Her connection to her daughters and her unique magical abilities make her a truly memorable character. Shella is cheerful, playful, and often gets interested in what Basara Toujou and her daughters are doing. She has a lively and open personality, and sometimes acts just as curious or even mischievous as Maria Naruse. She is not shy about magical contracts or awkward situations, showing she is very confident with who she is.

Brynhildr was one of the most powerful Valkyrie, who hunted down and collected evil spirits for the Chief God, but sacrificed herself to seal an S-Rank Evil spirit, turning into a demonic sword sealed in the Japanese Branch of the Hero Clan being wielded by Basara Toujou. Not much is known about Brynhildr's personality, but she was once known as a rather dutiful Valkyrie who served the Chief God without question, even sealing away a dangerous Evil Spirit at the cost of her body and soul. After becoming a demon sword, Brynhildr has shown to prioritize her life as she chose Basara as her host to keep from being erased by Banishing Shift, and again after Ornis tried killing Basara.

Liala is the daughter of a prestigious Duke Household and the adoptive older sister of the current Demon Lord, Leohart. For some reason, she underwent an atavism, which made her look more like someone from the God Race. From her first appearance, Liala has almost always been shown smiling, possessing a playful, gentle, curious, and childish demeanor around several high-ranking people more so her adoptive younger brother she is noted to love more than anyone else. Despite her childish demeanor, she is highly perceptive and intelligent, as well as serious at times being able to figure out that both Jin and Basara were planning on killing the Demon Council if she hadn't done so herself, as well as the latter being responsible for Belphegor's death, however, she plays all this off as being apart of her woman's intuition. Liala can be ruthless against anyone she views as a threat to Leohart or makes attempts on his life to the point of being referred to as the Massacre Demoness, which was further noted as Leohart stated she wouldn't hesitate to turn her anger toward the people responsible for the birth of Shiba, such as the Hero and God Clans if he died during the events of Volume XI.

Nanao Tachibana is one of Basara's close friends in school aside from Yahiro Takigawa, although she is first portrayed as a male, she is, in fact a female student. She is part of Hijirigasaka Academy's student council member, as well as being revealed as a Half Vampire. Nanao is shown to be friendly and shy. She has spent a great deal of her life living in fear of herself and the Hero Clan, to the point that when she found out Basara was a hero as she attacked him while professing her innocence in the events surrounding Basara and Ornis. Also, she is extremely close to Basara, serving as his gym partner and this attachment intensifies after Basara learns about her Dhampir heritage.

Shiho Aikawa is a second-year student in Hijirigasaka Academy and belongs to class 2F. She is close friends with Mio Naruse, Yuki Nonaka, and Chika Sakaki who she is seen eating lunch with most of the time. During her first-year as a student in Class B, Shiho and Chika was both inadvertently dragged into Zolgear's plot to capture Mio, when Zest took control of her during the events of Volume III. Shiho is a friendly person who talks with her friends, Mio, Yuki, and Chika.

Chika Sakaki is a second-year student at Hijirigasaka Academy. She's best friends and classmate of Mio Naruse, Yuki Nonaka, and Shiho Aikawa as the four are often seen having lunch together. In her first-year as a student in Class B, Chika and Shiho was both inadvertently dragged into Zolgear's plot to capture Mio, when Zest took control of her during the events of Volume III. Chika is shown to be a kind girl as shown in a number of her talks with her friends such as Mio, Yuki, and Shiho.

Rukia Lucia Naruse is a supporting character from the anime and light novel series The Testament of Sister New Devil BURST. She is a female demon affiliated with the Moderate Faction, serving as a close aid to Ramusas, and is the older sister of Maria Naruse. Rukia is a stoic, disciplined, and responsible maid who contrasts sharply with her mischievous sister Maria. Despite her serious and initially distrustful demeanor toward humans, she is deeply caring and protective of her family.

Celis Reinhardt is the childhood friend of Basara Toujou, Yuki Nonaka, and Kurumi Nonaka growing up overseas but at times came to the Hero Clan to exchange techniques and tactics. She served as the Hearing Officer of the Vatican tasked with representing the Vatican with regard to the meeting between Basara and the Village. After the battle with Shiba, Celis lives in the Toujou Household and attends Hijirigasaka Academy as an International Student. Celis is a rather professional and serious woman, taking her responsibilities serious, but she still values her childhood friends such as Basara, Yuki, Kurumi, and Takashi, being on their side after the Japense Hero Clan attempted to treat them as threats. Unlike the others, Celis didn't hate Basara for the incident that happened over six years ago and tried helping him.

Chisato Hasegawa, born Afureia is a school nurse of Hijirigasaka Academy and a former high-ranking god amongst the Ten Gods of the Divine Realm. Chisato is also the younger cousin of Raphaeline, making her the first cousin once removed of Basara Toujou. At first, she acts as a mentor for Basara for several issues in his life when he's protecting Mion Minami and the others from various threats unbeknownst to him, but she later develops genuine romantic feelings for him, leading the two of them into a secret romantic relationship for the majority of the series. She formed a formed a contract between her and Basara and spent a year in a pocket dimension created just for that purpose. Her secret would be revealed when she saved Basara and the girls from Shiba Kyoichi, and told them of the Master-Servant Vow. Chisato was initially shown to be a wise and caring person proven to be almost completely shrouded in mystery. Throughout the series, she is noted to give Basara advice when he's caught up in situations that he couldn't solve on his own, as well as being someone capable of giving advice to other students, despite her being a nurse. She mentions that female students sometimes ask for advice about men (despite not having experience with a man). She has proven to have a strong attachment towards Basara, more so than the other students, to the point of asking for assistance in taking off her swimsuit and inviting him to her apartment for dinner. Chisato even started to make more sexual advances toward Basara and progressed to them having a secret relationship, accepting all of the advances made toward her without refusing. Despite being calm most of the time, she can be jealous of all the other girls who are in love with Basara as stated several times when he already spends more time with them rather than herself

Kurumi Nonaka is a member of the Hero Clan in Japan and the younger sister of Yuki, making her the youngest child of Shuuya and Kaoru Nonaka. She is an elemental master with a strong connection with spirits. At first, she appeared as one of the three sent to eliminate Mio, upon joining the Toujou Household, she becomes one of Basara's girlfriends. Kurumi is first shown to be very protective of her older sister and is also dutiful to her position as a hero. She also held no sympathy for Basara after the incident from five years ago, even calling him a traitor due to a mixture of the grief that it caused her sister, protecting the daughter of the Demon Lord, despite her being initially being close to him thinking of Basara as an older brother. She didn't hesitate to attack Basara in their battle against one another, then even showed to be insulted after he didn't take her seriously as he used to blunt end of Brynhildr to attack her. However, after Basara saved her from falling and hearing his suffering as he slept upon being knocked out by Yuki, she realized that he was suffering most after the incident out of the three of them. Kurumi started referring to him as "Basara-oniichan" as she had done in the past. She quickly developed a close bond with those in the Toujou Residence, but Kurumi is described as having a certain innocence, which riles up the succubus instinct in Maria into the depths of depravity becoming both a sexual plaything for her yet close friend. This close proximity, however, it also forces her to confront her long-standing feeling for Basara, which at first caused her to shyly use her spirits to simulate a muted dream for her a couple of months in a row. Kurumi later starts to become jealous about the fact that Mio, Yuki, and later Zest were able to make a Master-Servant Pact with Basara. Then the actions of the Succubi Lucia and Shella forcibly brings these feelings to the surface by cursing her and making Basara subdue her, repeatedly.

Zest B. Steward is a Demon that was created by and served Zolgear as his maid, she was sent by the Demon Lord Faction to serve as a scout alongside Lars. After the defeat of Zolgear, she temporarily served Sheila, the mother of Lucia and Maria having lost her original purpose and complied with the demands of the Moderate Faction. She ultimately serves Basara as she is tied to him in the Master-Servant Contract in a manner similar to Mio and Yuki but instead uses Sheila's nature. Zest is known to be fiercely loyal to her master. Cold and calculating, she never goes against her master's wishes. She, however, does not practice blind obedience and would advise her master should she feel that her master's decisions would result in undesirable consequences. This trait was exemplified when she advised Zolgear to refrain from raping Mio Naruse before her powers are completely extracted, as this will result in the loss of her powers which was essential if Zolgear were to overthrow the current Demon Lord Leohart. Her previous life makes her quite naive in morals, save for the servitude she was taught by Zolgear. This also quite often puts her in the crosshairs of Maria's succubus antics. Cold as she is, it doesn't mean that she doesn't have the capacity to have and feel emotions. This was first implied when she scouted the Toujou Residence during Basara's subjugation of Mio and Yuki Nonaka, wondering if Zolgear would love her as Basara would to his servants, or what if Basara were her master instead, although she asserted that she felt "no jealousy whatsoever". Her emotions became even clearer as she served Sheila, and later, Basara. It appears that she has a complex and constantly wonders whether she is useful enough to her master. She is also revealed to have a humble and meek personality when she is not fighting her master's enemies and is the only one amongst those bounded by the Master-Servant Contract to address Basara with the suffix "-sama".

Yuki Nonaka is a member of the Hero Clan in Japan, a childhood friend of Basara Toujou as well as the older sister of Kurumi Nonaka, making her the first child of Kaoru and Shuuya Nonaka. She is the initial observer watching over Mio Naruse as such she attends the same school, as well as being the class representative of her class, where Mio and Basara are. She later becomes one of Basara's lovers. Yuki is usually calm and gives off a very relaxed air, and is proper of her actions. However, she displays a different side whenever she's around Basara, like being intimate and displaying loneliness. Being a member of the Hero Clan who is sent by "the village" to watch over Mio, she is dedicated with her role, even going as far as telling Basara that she won't show mercy on killing the future Demon Lord if she makes her surroundings dangerous, even if Basara would come to hate her. First time learning of Mio's relationship with Basara, Yuki considered her to be a hindrance to his peace. But later on, she and Mio become close friends and also rivals in love albeit the rivalry became even more intense after she started to live with Basara. Yuki often gets jealous of when Mio (or other members of the harem) get to do things with Basara that she didn't and wants to do, and normally she takes the initiative of doing them herself or having Basara make up for it later. Because Yuki wasn't born talented, she values repetition and working hard when it comes to training as well as schoolwork. She's the type of person that believes anyone can reach her level as long as they are willing to put the work in to reach it.

Maria Naruse is a succubus and Mio Naruse's Guardian after the death of her adoptive parents. She is the daughter of Sheila and the half-younger sister of Lucia. It is later revealed that Maria is also the daughter of Wilbert, thus making her the niece of Sapphire, the younger half-sister of Mio, and the cousin of Basara Toujou. Maria is rather childish and likes teasing people no matter how serious the situation. She prides herself on trying to make light of most situations to counter the seriousness of Basara and the rest of the harem. She's often referred to as being the "problematic" member of the family due to how brutally honest she is about her feelings and desires. Being a succubus, Maria naturally feeds off of the lewd situations. In the first half of the series, she's primarily scheming and watching Basara engage in lewd acts with the rest of the harem in order to deepen their bonds. However, in the second half of the series, she becomes an active participant in the lewd situations, and even discovers that she's a bit of a prude and a masochist when it comes to others watching her engage in lewd acts with Basara. Outside of being a prankster and tease, Maria has a vulnerable side. As a child, she was repeatedly told that her mother's appearance was her fault. Maria then internalized that and spent the majority of the series blaming herself for her mother's condition. She also has moments, quite a few moments, where she's brought to tears and questions her worth and role in Toujou Residence. According to Sheila, although Maria is an experienced succubus, she's inexperienced when it comes to being a girl. She's in love with Basara, as with the other members of the harem. However, unlike the other girls, Maria often pushes her feelings down, and rationalizes them by saying "She's supporting Basara deepening his bonds with the other girls". She has some moments, though where she puts her feelings first and rewards herself with quality time with Basara.

Mio Naruse is the daughter of the previous Demon Lord, Wilbert and a human named Ashe thus making her a humanoid demon, as well as the niece of Sapphire. After the "death" of her father, Mio inherited his power and was raised in the Human World as the daughter of two unnamed demons as a human. Though after the death of her adoptive parents, she was later saved by Maria who revealed the truth about her lineage. Both later came up with the ruse where their "mother" would be marrying Jin Toujou and become the stepsisters of Basara Toujou in an attempt to make their house their base, while father and son would have their memories altered and move elsewhere. However, this plan failed after both of them were revealed to be banished members of the Hero Clan, but Mio and Maria continue to live in the Toujou Household. Mio is shown to be a calm and quiet person, especially around those she isn't close to, but she's rather talkative around her closest friends such as Chika and Shiho. It is noted that she can be rather quick to anger based on the situation, mostly if someone close to her is threatened, insulted or harmed, first shown when she slapped Takashi who didn't understand it was Basara who was suffering more than anyone after the incident from six years prior. Mio can also become enraged if she believes someone important to her has been killed to the point of wanting to eliminate the person responsible herself. Despite being referred to as a dangerous threat by the Hero Clan, Mio is rather caring, honest and compassionate. She hates lying for the most part to those who have helped her, as shown when she and Maria lied to Basara and Jin about their marriage to use their home as a base, even choosing to erase their memories so they wouldn't get involved in their problems anymore. Although Mio attempts to act brave and calm all the most, deep down she is afraid about her current situation since she has lived most of her life as a normal high school girl.