Koyomi is one of Fate's many pactio partners and like the rest of the girls she has romantic feelings for him.
One of Fate's Ministra Magi who can control time with her pactio Horaria Vortix. Compared to the other girls in Fate's harem, she is more childish, emotional, and easily embarrassed. She has a very strong loyalty to Fate, and perhaps even romantic feelings, since he rescued her from the brink of death.
Fate attempted to send her off to Ariadne Academy in the past, but she refused to enroll, desiring to stay by Fate's side to help him complete his plans. In contrast, she has a poor relationship with Dynamis, Fate's other teammate, who is very annoyed by her and calls her a cat.
She first appears with Tamaki to have negotiations with Rakan, but this was a trap to stop time and lure Rakan, Chamo, and Konoka into Tamaki's artifact. After she has her panties stolen she fought against Rakan, lost, and escaped. Later she and the other girls, except Tsukuyomi, appear on the roof of the Government-General building to battle Rakan again at the Governor's Ball. She then transformed into a wild black cat to fight Rakan, but she was easily defeated.
She appears with the other Fate Girls to battle Ala Alba in the Gravekeeper's Palace, using her artifact from a distance for safety. She is able to trap Negi by using her artifact to freeze him in place. However, she was subdued when Makie used her Liberum Lemniscus to strip her of her artifact, and was attacked by Ku-Fei, who was able to reach Koyomi with her staff. Koyomi soon returned, however, transforming into her Beast mode and attacking Ku Fei, who was defending the less experienced fighters and non-combatants. She manages to break through, and attempts to strike Ako down, but Yuuna, her abilities boosted by Ako's drug, shoots her before she can do so. Yuuna, Makie and Luna drive her back, and she is last seen having rejoined the fight against Ku Fei.
- Horaria Vortrix: Koyomi's artfifact. It can control time but it's more detailed effects are not known as Rakan does not allow it. Since it has been shown to stop or speed-up the flow of time, it is a very powerful artifact; however, Koyomi is easily embarassed and is easily distracted from using her artifact when upset.
- Beast transformation: Koyomi turns into a cat-woman covered in black hair. She also combines it with her Horaria Vortrix to boost her speed. Her main offense in beast form appears to be her super-sharp claws.
- Her codename, Koyomi, means "calendar", which references her time-controlling artifact. Her true name is unknown.