Koyuki as seen in Episode 4
|Voice Actor||Revy Moonshine (SFA4 to SFA18)|
|First Appearance||Episode 4|
Episode 14 revealed Koyuki's background: she was raised in a hidden ninja village in the forest with her friend Mukuro. She rescued Zeroro after he broke his leg during the failed invasion of Earth, leading him to join the ninja clan and rename himself Dororo; he shortly after played a lethal game of hide-and-seek, causing their village to disband from all of the casualties. Koyuki and Dororo now live just outside the city in a hut.
Koyuki has attempted integrating into standard society by enrolling at Natsumi's school, but she can't keep her skills under wraps. She has developed sudden hatred for, and attacked, inanimate objects.
Koyuki notably has the strongest friendships with Dororo and Natsumi; she is one of the few characters who cares for Dororo, and Natsumi considers her to be the only real friend she has. However, because every girl at Natsumi's school is in love with her, Koyuki ends up being no different, although she did manage to take Natsumi on a date. Natsumi is finally willing to begin a relationship with her in Episode 18. They are seen together in Episode 23 and The Sgt Frog Abridged Movie.
She had a close friendship with fellow ninja Mukuro before their clan disbanded.
- Koyuki was the first main character who was not voiced by a series writer to have and maintain a voice actor (Revy Moonshine) until their debut appearance. Up until then, Natsumi, Giroro, Dororo, and Momoka all had voice actor change-ups before they ever appeared, some going through multiple VAs.
- Revy Moonshine stepped down from voicing the character following Episode 21, having stopped voice acting in the interim between episodes. ShadowKitten91 was asked to fill the role, but was unable to record due to a house fire. Team friend Ches instead filled the role from thereon-out - Ches had previously appeared as Omiyo.