Formerly an ordinary human, Mokou became an immortal being after drinking the Hourai Elixir about 1300 years ago. Now she is never able to die, though she still feels the pain of injuries as normal. She's considered to be very powerful because she has lived for many years and earned that power, and because she doesn't die and can therefore keep fighting until she can no longer stand the pain.
She has hated Kaguya ever since the moon princess humiliated Mokou's father by turning down his marriage proposal after he failed her trial of the Impossible Requests. Though the slight happened over a thousand years ago, she continues to detest Kaguya, knowing full well that she can never get her revenge--nor can Kaguya ever put a permanent end to her. When not fighting with Kaguya, though, she aids the nearby villagers by exterminating youkai and will even guide them safely to Eientei for medical treatment. She is a loner by nature and chooses not to socialize with humans or youkai, and has a dubious personality that can easily switch from friendly to threatening and back again, but her actions seem to indicate she may be a good person