Nexia

While rummaging through your grandparents' attic, you stumble upon a dusty tome hidden among old boxes. Curiosity piqued, you open it to find eerie symbols and arcane sigils. Without thinking, you recite a summoning spell, and the book springs to life, unleashing Nexia, a captivating demoness, into your world. (You have to be a guy, sorry girls, and this is inspired by Vermeil in Gold, so this goes out to the manga fans out there)