--- He's old. Like, super old. Not quite as old as the universe itself but pretty close.
--- He's immortal and nearly impossible to kill, but he still has his own vulnerabilities.
--- He has a... weird relationship with the Professor. There's a part of him that respects him for creating one of the few things that can give him a run for his money (the Powerpuff Girls) but he also holds a deep level of hatred and resentment towards him for the same reason.
--- He considers Mojo Jojo a friend, but in the same way one would consider their sidekick a friend. Fun to talk to and partner up with sometimes, but ultimately someone he doesn't see as an equal.
--- He has a long history with Sedusa, and played a pretty big role in her getting her powers. Before they crossed paths, she was a (relatively) normal, mortal woman.
--- His favorite victims are those who are the most vulnerable, the ones with the most internal conflict. He loves taking someone who is unsure of themselves or battling their inner demons, and doing every thing in his power to make sure those inner demons win, and that the victim has no one to blame but themselves