Everybody has a protection spirit. Their behavior has a direct influence on the person they protect. For instance, if protection spirit A falls in love with protection spirit B, then host of A will fall for host of B (synchronized effect).
Karin has Hiriji as a protection spirit and that's why he doesn't have friends. It all changes when Karin has a little incident and gets help from his sensei (teacher), Takatsugu, who's protection spirit is Nagi. Sensei is the school nurse and can see, hear, and touch protection spirits and (the same as Nagi) was a playboy 'till he meet Karin (Hiriji).