I would say the best character development comes when a character is well-defined throughout the course of the series. This way, they’ll have a real struggle/issue they can actually work on. For some reason, Marceline The Vampire Queen came to mind so I’ll just use her as an example. When she’s first introduced to Adventure Time, she’s known as a stereotypical, scary, and pretty mean vampire. However, once she befriends Finn and Jake and starts going on more adventures with them, the audience gets to see a lot more layers to her. She’s a really good musician, a mischievous but loyal friend, and her relationship with Princess Bubblegum and other stuff in the series definitely made her pretty mysterious. Overall, writers can create characters that are engaging and believable by giving their characters charming/fun/likable personalities and adding depth. While good personalities/cinnamon roll characters are great, it’s gonna mean nothing if they don’t have any internal struggles, goals, motivations, philosophy, etc.