There are so many examples of this character archetype. It makes for a great discussion. Head over to Story Empire to read this fascinating post by C. S. Boyack. Here’s some of my examples: Gollum and Frodo (the former on his own too), Two-face, Joker and Batman, Spiderman and Anti-Venom just to name a few. I’ve got a main character who struggles with this at some level.