Rei Nishina is Keiichiro's ex-girlfriend who appears in episode 14. She's only in the anime version of Tokyo Mew Mew.

Rei is a woman with wavy medium brown hair that reaches a little past her mid back. Most of her hair is loose, but the last few inches are tied in a ponytail. Her eyes are a very dark gray or black.

Throughout most of episode 24, she is seen wearing a lab coat over a plain, simple outfit. Flashbacks show her in more casual clothes while not working.

Rei is a Lepidopterist, someone who researches butterflies, and uses clues on their population to find out how the environment in Tokyo is changing. The job keeps her very busy that there are times when she can't go out for fun but she loves her work due to the butterflies, wishing that she could be just as free as they are.