He was born that way. But the people in the city he frequents don't quite believe in the urban legend, so he grew up not quite knowing how to scare/haunt/kill people. He even forgot his queues to do so. XD So he sits in the corners of alleyways with his mouth covered simply staring at people who walk by. The dude is rather awkward and is more fascinated by people than he should be. He has a tengu-masked friend that tries to tell him what to do in order to be a good slit-mouth, but he gets ignored. XD The slit-mouth would rather follow a human around that he likes and stare at them more than kill them. He has no idea that it's creepy to stare at people. XD
What a neat story! I like it ^^ Though I don't think he and Han would get along xD Han is all about killing, she could join the demons with their quest to take over the world but Han thinks it's cowardly to have a group.