Kokushibo is the mafia leader's right hand man and a mafia boss himself. He takes the role of creating deals with people, loaning, etc. He'll send his goons after anyone who doesn't pay him back, doesn't fufill their end of the bargain, or anyone he just overall thinks is too disrespectful. One thing you should know is that you should never interrupt him while he is busy, especially not if he's in a meeting, training or dealing with somebody for a deal.