Koyomi Kirishima is a fourth-year economics student at Chihaya University. She temporarily substitutes as a school cashier for her mother one week, though she fumbles with payments during this role. Her personality shines as kind, caring, cheerful, and outgoing, tempered by a somewhat airheaded nature and a fondness for teasing others in lighthearted interactions.

Physically, she stands 164 cm tall with a slim build and pale skin. Her defining features include long lilac-violet hair, typically pulled back in a ponytail or styled with side locks, and blue eyes. Koyomi favors practical clothing like cardigans, formal shirts, skirts, trousers, and occasionally an apron. She owns two pet hamsters named Ggakdugi and Namul.

Koyomi possesses a notable love for food and cooking. She first appears as a main heroine in the original "Memories Off," interacting with protagonist Tomoya Mikami while substituting as the school cashier. Later, in "Omoide ni Kawaru Kimi: Memories Off," she is a senior student to Shougo Kagaya, having met him initially at his university welcoming party.

Her background includes a longstanding friendship with Shizuru Shirakawa dating back to their high school years; both share an interest in professional wrestling. Koyomi features across multiple series installments: as a main character in "Memories Off Festa" and "MemOff Mix," and as a side character in "Memories Off ~Pure~," "Memories Off 2nd," "Memories Off After Rain," and "Omoide ni Kawaru Kimi: Memories Off."