Princess Bubblegum is the 18 year old ruler of the candy kingdom. She is a major character in it and in the episodes she is either needing saving or being cool and saving Finn and Jake.
The princess seems to be an orphan. There is no mention of Bubblegum’s parents in any episode. If the princess got any younger, like in “Too Young”, then the candy kingdom would be handed over to the arrogant Earl of Lemongrab, but that has only happened once.
Princess Bubblegum has no boyfriend nor husband, but in “Ricardio the Heart Guy” a heart man named Ricardio impresses the Princess enough to marry him, although Ricardio did turn out to be a living, breathing version of the Ice King’s heart.
The Princess trusts Finn to the death, and in “The Enchiridion” he saves her life so she embarks him on a terrifying quest to find a magical book called The Enchiridion.
Finn and Jake refer to Princess Bubblegum as “The Princess”, as she is the main princess in the series, but Marceline the Vampire Queen refers to her as “Bonnibelle” as that is her real name.