Koi Chigusa is one half of a pair of twin sisters in the anime Futakoi, the other being Ui Chigusa. Their father is a veterinary doctor, and the sisters help him run his clinic. Koi has brown hair and brown eyes, wears round glasses, and styles her hair in a braid. She is a student who balances her studies with work at the clinic. Her personality is marked by a helpful and diligent nature, as she is dedicated to caring for animals and supporting her family. In the story, she and Ui are introduced when the protagonist Nozomu Futami brings his goat Billy to the clinic after the animal swallowed a fax. Koi assists in diagnosing and treating the goat. Her role in the series is that of a minor supporting character who appears in a specific episode. She shares a close relationship with her twin sister Ui, working alongside her in the clinic, and she is supportive of her father. She does not undergo notable development over the course of the series. Her notable abilities involve practical veterinary skills acquired through her family's clinic.