Formerly a Lunar Rabbit named Rei'sen who fled from the Lunar War, abandoning her comrades. It's not known whether she did it out of cowardice or something else. She was found in Eientei by Kaguya and became her pet and Eirin's student. Changing her name to "Reisen", and receiving the nicknames "Udonge" from Eirin and "Inaba" from Kaguya (though that's what Kaguya calls all rabbits), she now loyally serves and protects her master and teacher. Being a Lunar Rabbit, she is gifted with the madness-inducing eye, though Phantasmagoria of Flower View reveals that her eyes actually control waves in general, including mind waves and electromagnetic waves.