Kirino Ootori is a freshman at Sakashitamon Academy as well as a tenant at the girls' dormitory. She has what Hachibei considers to be an "ideal voice". She is descendant of a ninja family and master of the Shuusui ninja style. She loves to sing, but has to hide it because her grandfather's disapproval of it getting in the way of her ninja training. For this reason, she loves to go to karaoke by herself.

She has the ability to sense another person’s killer instinct. She often threatens to kill people that annoy her yet this usually has no effect on Hachibei for he enters a bliss state by simply hearing her voice. Her 8 year old cousin Ai once tried to take her life (to no avail) but in the end they made up. She normally protects her cousin from Hachibei's perverted actions because they found out that Ai has Hachibei's "ideal hands". It was revealed that Ootori looks up to Hachibei because of his "unbelievable strength" that rises throughout the story.

It was implied if Hachibei wasn't such a "skirt-chaser", Ootori would like him more.[14] This as well as joining up with Sakurako and Yukari to get Hachibe back. She has an extreme hatred towards anyone making fun of the ninja way, which provoked her wrath when Yukari cosplayed as one. She tries to hide her growing feelings towards Hachibe, though not to well: such as making Obligatory Chocolates for him on Valentines Day, yet made a vast amount for him. She later becomes an idol star and leaves the house however, not before showing her feelings for Hachibe. She returns in the final chapter, commenting on how he was a strong boy and sung to him her newest song with the voice he loves so much.