Reisen is a part of the
Speckled Sky.
Reisen the lunar rabbit. Reisen the straight man. Reisen the refugee.
Reisen Undongein Inaba fled from the moon during the great Lunar War, and took refuge, along with other lunar rabbits, with the fugutives Kaguya Houraisan and Eirin Yagokoro in Eientei, an old fashioned Japanese mansion hidden deep in the bamboo forest. Under their care, she protects and works for the two, selling the medicine that Eirin cooks up.
It's said that those who look into her eyes know insanity. She just naturally knows how to bend light and sound to create illusions that can fool anybody. Along with her proficiency with guns, this should make her quite the adversary, if she wasn't sane.
You see, Reisen isn't the most... Let's say 'unstable' of the kind you can find in Gensokyo. She's calm, collected, and level headed. Unfortunately, those aren't traits you'd want to have if you wish to survive in Gensokyo. She plays straight man to the rest of the denizens of Gensokyo, and is frequently the butt of their jokes, among other things butted into her.
Under Eirin's care, she frequently has to test whatever new medicine she has concocted, and more often than not, it ends up with Reisen missing teeth or on fire. Even the medicine that Eirin concocts for more underhanded purposes gets fed to the rabbit, whether she likes to or not. There's no need to worry, though. Reisen has managed to mostly suppress some of the long term side effects of that mood enhancing drug.
But she just takes it with a smile. After all, someone needs to be the most level-headed around here.