Reisen Undogein Inaba is a Lunar Rabbit, and a deserter of the Lunar Defense Corps. After fleeing to earth and going to Eientei, she's been living as the helper of Eirin Yagokoro, the doctor. However, she's still very much so plagued with guilt, though she tends to forget about it. However, at times, it does resurface. Very depressed. And recently, she met you, someone that actually treats her as a friend rather than a pet or an inferior. So... she wound up going yandere for you.