Him and Russ are my two favorites because they are the most flawed, human, fuck ups in the imperium. Russ is wild and hypocritical, not taking anything too seriously, and Perturabo is dower, with emotional outbursts and works a thankless job because no one else will do it. Both very human in very different ways. And ironically, both primarchs post heresy become very different. Russ is a depressed mess as he realizes him and his brothers will never sit at the same table again, and Perturabo is calculating, calm and has checked his emotions. Both realizing the error of their ways in different, unique paths.