Yuika Koga is a petite first-year at Momosawa High School, standing 150 cm tall. Her blonde neck-length hair and green eyes reflect her heritage: one-quarter Caucasian from her British grandmother on her mother's side and Japanese from her father. Often described as angelic and cute, she wears her uniform with a signature green hair ribbon and knee-length white socks. Initially cold and distant upon entering high school due to ostracization over her foreign looks, Yuika isolated herself in the library, avoiding interactions and having no friends. This changed when fellow student Keiki Kiryuu persistently engaged her during her library shifts. His consistent kindness helped her overcome her loneliness, warming her towards him and enabling her to form friendships within the calligraphy club. Externally, Yuika appears cheerful, playful, and approachable, especially around Keiki. However, she harbors a contrasting inner personality as a strict sadist with dominant tendencies. She desires to make Keiki her submissive slave, referring to herself as his master. This hidden nature emerged fully when she confessed her true feelings to him after a date, revealing her fetish as the polar opposite of Sayuki Tokihara's masochism. Yuika becomes jealous when Keiki interacts with other girls, particularly Sayuki, whom she derisively calls "Witch" due to their rivalry over Keiki and physical differences, including Yuika's self-consciousness about her small chest compared to Sayuki's larger bust. Yuika serves as an assistant librarian alongside Keiki and belongs to the calligraphy club. She possesses a strong passion for literature, aspiring to become a picture book author and actively writing a story titled "The Shy Princess." Her artistic talents also extend to calligraphy. Throughout the series, Yuika engages in mischievous and dominant behaviors toward Keiki, including attempts to "train" him. One extreme incident involved stuffing her underwear into his mouth, causing him trauma. Her romantic feelings intensified after she interpreted a casual remark from Keiki as a marriage proposal, leading to an embarrassed but earnest acceptance. Her rivalry with Sayuki escalates into frequent clashes, though their confrontations inadvertently satisfy each other's fetishes—Yuika's sadism and Sayuki's masochism—providing Keiki temporary reprieve. Yuika lives with her family, with specific details beyond her mixed heritage and grandmother's passing undisclosed. Her development centers on transitioning from isolation to forming connections, driven by Keiki's influence, while navigating her complex romantic desires and ongoing conflict with Sayuki.