Kirigaya Hitsugi is a girl who has the body of an elementary school student. She has a bad sense of direction, as demonstrated when she gets lost on her way to Myoujou Academy. Although Hitsugi looks like the horribly unreliable type, she also has her reasons for being in the Black Group. She is also quite attached to Chitaru, as she is a nice and caring person.
In chapter 18, it is shown that she has a hidden gun within her stuffed teddy bear, proving that she is not just some small incompetent girl. It is also revealed that she is somehow affiliated with an organization that killed Chitaru's old teacher, whom the latter is trying to avenge.
In Episode 6 of the anime, she stabs herself in Chitaru's hands by plunging herself to the knife Chitaru was holding against her. She survives the stabbing and is seen tending to Chitaru in the hospital during the end credits.