Finn, and Jack have adventures in the land of Oog.
Finn the Human - The Main character. Finn is an adventurer.
Jake - Finn's adopted brother, and canine companion.
Marceline, the Queen of Vampires, and Princess of Demons - Marceline is a half demon who became a Vampire during her teenage years. She is one of the few survivors of the Great Mushroom War. She killed the Vampire King, taking his throne for her self. She, and her previous boyfriend, are the only vampires (aside from Marcel) to appear in the series. Despite being a type of monster she really does have a heart, and feeds on shades of red.
Ice King -
Princess Bubblegum - Finn's romantic interest, she is older then him, and thus does not see him in the same light. She is ruler of the Candy Kingdom.
Rainbow Unicorn Princess -
The episode What Was Missing, though not explicitly stating it, seemed to many to imply (though a blush on Marceline's part) that the female characters of Princess Bubblegum, and Marceline had a sexual relationship some time in the past. A request by the staff on input on such a relationship between the two characters lead to a mess of issues after graphic fanart, and fan fiction began to circulate. The staff, merely wanting the relationship to be childish in nature, dropped the idea after such exploson of input, both positive, and negative, caught unwanted attention.  The episode still runs, with the canon understanding being that Marceline envies the Princess, thus finding out that the Princess prizes the shirt Marceline gave her caused her to be flattered, thus blushing.