There are many characters in this show. The main characters are
Finn: The last human kid. He dreams of one day being a hero
Jake: a magical dog who can stretch into alot of different form, but tends to be pretty lazy
Princess Bubblegum: Princess of the candy kingdom. She is normally to busy for fun and is the most logical of the characters.
Marceline: A 1000 year old vampire rocker who lived before the mushroom war and constantly has issues with her past
Ice King: A character who wants nothing more then to gain a bride and keep Finn and Jake out of the way. He has the darkest past of any of the characters. and that's basically the basics