Koyomi is an androphobic resident of Seiren. Her phobia is only made worse by being one of Kazuharu Fukuyama's favorite targets; in the manga, she keeps a diary of how many times he has accosted her. Originally sent to Earth on a mission from the Space Travel Agency to "retrieve" Miharu Sena Kanaka, she later moves into Yukinari's house on a prolonged mission to find a husband for Miharu's sister Maharu; however, her fear of men makes the act of even talking to a man nearly impossible.

Koyomi's favorite pastime is reading, but she has a natural talent for table tennis, having won a tournament for a similar sport on Seiren.