Betty Grof

Betty Grof is a human woman revealed to have once been the fiancée of a man named Simon Petrikov, who would much later become the Ice King. She and Petrikov were in a committed relationship until Petrikov obtained an enchanted crown, which gradually began to alter his body and warp his mind.

History
As revealed in "Betty," when Petrikov was wearing the crown for the first time, he chased Betty through a town while shooting ice bolts and asking "WHERE IS MY PRINCESS?" As he had said "Betty, my princess" in "Holly Jolly Secrets Part II," this likely meant he was looking for her.

It is not clear whether or not Simon had called Betty "princess" prior to wearing the crown, or whether or not this is a name he affixed to her (much as he began calling Marceline "Gunter") as a result of names whispered to him by the crown.

In "Holly Jolly Secrets Part II," Simon claimed he never saw Betty after his initial time wearing the magic crown (although he may not remember if he encountered her again as Ice King), and he records on video his desperate wish to regain his sanity one day so that he might apologize to her and she might return to him.

In "Betty," Simon regained his sanity after having his ice powers stolen by Bella Noche. Back to normal, he enlisted Marceline, Finn, and Jake to help him create a portal to the past before he drove Betty away, using Hambo as a catalyst. With the portal open, he took the moment to attempt to apologize to Betty, but instead she leapt into the future to be with him, thus explaining why Simon never saw her again in the past.

It was only after crossing through that she learned that Simon was dying. With Bella Noche having stolen all the power from the wizards of Ooo, the magic in the crown that kept Simon from aging was now gone and Death would soon catch up to Simon. Betty took it upon herself to get Simon's powers back to save him from dying. She promised that she would try and find a way to keep him alive and free from the crown's curse. Arriving at Wizard City, Betty defeated Bella Noche (using the advantage of surprise) and restored the powers to the wizards as well as the dying Simon.

She is later seen observing Simon, now the Ice King once again, in his castle (she is noticed by Gunther) She then exits on a flying carpet to begin her search for a way to free Simon from the crown's influence while keeping him from dying.

Personality
Betty doesn't make an actual appearance in the series until the episode named after her. In the episode, she is shown to be very loving and devoted to Simon. She is shown to be bold and quick-thinking, as she went to face off against the creature Bella Noche by herself in an attempt to keep Simon from dying. She is shown to be a decently capable fighter, managing to defeat Bella Noche in two hits (though she did have the element of surprise).

Like Simon, she appears to be quite educated in magical artifacts as she was confident she could return Simon to normal. She also co-authored the book "Mystic Rituals And Their Space Time Applications" with Simon.

Appearance
Betty is an adult woman appearing to have pinkish skin and auburn hair. In a picture on Simon Petrikov's wall, Betty is seen wearing stylish glasses and a green turtle neck. In another picture, Betty is wearing a lab coat. This might suggest that she was in a field of study, such as science or medicine.

Trivia

 * Simon referred to Betty as his princess while he still possessed some of his sanity.
 * Adam Muto stated that the figure in the picture with Simon and Betty is "some sort of cryptid." A cryptid is a creature that has been sighted before but whose existence is unconfirmed by science (Big Foot).
 * Pendleton Ward and Rebecca Sugar have stated specifically that Betty and Doctor Princess are not the same person.
 * Betty's last name is an homage to, a psychiatrist who's one of the founders of the field of.
 * In the older episodes where she has a brief appearance, her glasses are rounded, but in the episode "Betty," her glasses were pointed at the ends.