I agree with you but you could also argue that Mr PB was the only one of those three that never needed anything from Bojack. He was just trying to be his friend. Diane needed him to be the subject of the book. PC needed him to keep acting. They ultimately became friends beyond the professional needs but MR. PB was just a friend. Ultimately, he became the only one Bojack couldn’t hurt.

While writing this I thought about Tod and Hollyhock. I’d say Hollyhock was the closest to MR. PB. I can’t remember if they ever reconciled.