I don't think they were working together from the start. The yakuza dude said he couldn't find Kuze until he approached himself. ( Or found him when he took him home ) He just let him use the evidence he collected and Kuze had no idea bout this. They started working together after their first sex when the dude said him to take to the police but Kuze declined.
They definitely didn’t work together from the start, I’m quite sure about this. In the first chapter Shimojou found Kuze’s police identity card while Kuze was showering and said “You are really a boring guy Kuze-kun” like..if they work together this scene wouldn’t make sense at all. It’s like Shimojou just found out Kuze is the police from that scene. And in the first chapter too, before Kuze approach Shimojou he read the report about the group and saw Shimojou in the photograph, He said “found you”.
This guy is getting on my nerves. Leave the poor boy alone in peace !!!!!! I swear if they touch him I'm leaving this comic. I see no story in this anyway.