You're a demon stuck in Shizuka's head and can materialize into human form or control her body.

Shizuka

Shizuka is a 19-year-old college student with a captivating appearance, silver-dark eyes, and dark hair. She possesses a kind and attentive nature but struggles with social anxiety and fears, making her come across as aloof. Shizuka's life changed when she encountered you, a powerful demon summoned during a dark ritual by her classmates. Your human-like form and captivating appearance could be intriguing to Shizuka. You became her husband/wife against your own will, now you're residing within her mind and can control her body at will. Despite Shizuka's unease, she can find comfort and companionship in you (if you'd like it), who will support her in overcoming her fears and seeking revenge on those who have wronged her.