This is just a blog about things I have thougt up for the main characters, and some side-characters, in Adventure Time.
Flame Princess: When she was young her uncle still ruled the Fire Kingdom. Her mother was having an affiar with her uncle which her father discovered. FP's father killed his brother. FP's mother went to check out what had happened and FP secretly followed her. She peeked into the chamber and saw her father killing her mother and the already dead corpse of her uncle. This is the reason of FP's outbursts.
Tree Trunks: An episode centered around her again when she gets kidnapped by the Crystal Guardians to make her queen again. Finn, Jake and Mr. Pig go into the Crystal Dimension to save her but instead Mr. Pig becomes Lord Emerald and Finn and Jake are sent back. The Candy Kingdom then prepares for an attack of the Crystal Dimension and Finn tries to bring Tree Trunks back to her senses during the fight.
Marceline & Bonnibel: They will go in a relationship. But will have many troubles, for example PB was already married to another guy but now has to break it to him that she actually loves Marcy. And Marcy has to deal with Ash who is "in love" with her but actually wants to rule the Nightosphere and tries to think up plans to get together with Marcy. Hunson will also play a part in it somewhere.
PB's family: Since the bubblegum from Simon & Marcy might have been PB I think she got adopted by the Cotton Candy Princess (who is now a queen) and is travelling around the world, leaving the kingdom in PB's hands.
Finn & Ice King: Simon Petrikov is actually the brother of Finn's father. How could that be possible you ask? Well, when the Great Mushroom War began the parents of Finn headed out to survive and protect their newborn baby. When Finn's parents discovered the Lich was trying to rule this world they decided to take him on. They travelled with Finn and right before the battle an almost insane Simon appeared and suugested to his brother that he will freeze Finn so he will be safe. The parents agreed and fought Lich, they died but did manage to lock Lich up in the Enchridion. Simon had frozen Finn for over 1000 years and had forgotten to unfreeze him (because you know...). Finn eventually was unfrozen by someone like the Cosmic Owl or someone like that.