Finn grew up from an immature little boy who always wanted to be known as an adventure to a mature young man who can chill, think, use his mental powers and still wants to be an adventurer but enjoys the things around him at the same time.
P.B grew up from having the mentality of always working and treating people as if they’re babies to becoming more chill, trusting and helpful.
Marcellene went from a rowdy party girl to an emotional girl who expresses herself through her music and isn’t afraid to let people in. She’s more of a grown woman now but she’s still fun.
Jake is still jake but with responsibilities
BMO is always BMO but we got to see more of him.
Flame princess and cinnamon bun have been hurt and grew from it and are trying to be the best they can be.
I love the character development of the show. No matter how serious and mature the characters get its still good and they add something cute for comic relief.