Betty Grof is a human woman revealed to have once been the fiancée of a man named Simon Petrikov, who would much later become Ice King. She and Petrikov had a very committed relationship until Petrikov obtained an enchanted crown, which gradually began to alter his mind and body.
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' before wearing the crown, or whether or not this is a name he affixed to her (much as he began calling Marceline 'Gunther') as a result of names whispered to him by the crown.
Simon presumably never saw her again after this (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 so that she might one day return to him.
In "Betty," Simon, who had been changed back to normal, was able to create a portal that linked to the past before he drove Betty away. He took this moment as an attempt to apologize to Betty, but instead, she leaped through the portal to be with him, only later to find out that he was dying. The cause was the being Bella Noche, who withheld all the power from the wizards of Ooo. When Simon opened the portal, it was revealed the reason she left him was that she fled when he became crazed and was attacking her with his ice powers, and then because she went forward through the time portal, explaining why Simon never saw her again in the past. She took it upon herself to get Simon's powers back and also stated that she was going to try and find a way to keep him alive and free from the crown's curse. Arriving at the castle, Betty defeated Bella Noche 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 exits on a flying carpet (possibly to begin her search for a way to free Simon from the crown's influence alive).
In "You Forgot Your Floaties", Betty retrieves the helmet of Grob Gob Glob Grod and goes to Magic Man's house to experiment to help cure Simon. While conducting the experiment Betty also tries to help Magic man become the new ruler of mars and free him of his sadness. However, due to Finn and Jake's intervention with the experiment, Betty receives all of Magic Man's powers, turning her into a wizard and turning Magic Man into Normal Man. In the process, Betty also loses her sanity.
In "King's Ransom", Betty gets some foxes to kidnap Gunter and would only give him back if Ice King trades his crown. When she gets the crown she reprograms it and gives it back to Ice King as a way to cure Simon.
In "Broke His Crown", it is revealed that Betty put a virus version of herself into the crown to reprogram it and bring back Simon. Only for it to damage the crown's systems and cause Ice King to become even madder. But thanks to Princess Bubblegum and Marceline the virus stops and lives romantically into the crown with the normal Simon.
She appeared again in the "Elements" mini-series as one of the main characters. She first appears in "Bespoken For". Betty is shown to have become more insane and started living in Magic Man's old house with Tiny Manticore. She then goes on a date with Ice King to help regain his lost memories but fails, much to her frustration. She was then captured by Patience St Pim and used as a power battery to unlock the full elemental powers of the elementals. However, this caused all of Ooo to be messed up in the process.
In "Winter Light", Finn, Jake, and Ice King rescue her from Patience and unfreeze her. She then comes up with an idea to fix Ooo. With the farm world Enchiridion, all they need were the gems of the other three elementals. Ounce Finn got all the gems Betty betrays Finn and flew off on her carpet with the book, the gems, and Ice King.
In "Skyhooks II", it was revealed that Betty never wanted to save Ooo but to go back in time to stop Simon from becoming Ice King and possibly stop the Mushroom War. But due to Ice King's intervention, the portal exploded and transported Betty to Mars now under the rule of Normal Man.
In "Temple of Mars" Betty is seen filling a giant hole with sand one grain at a time which she has been doing ever since she got to mars as part of Normal Man's (now King Man) rehabilitation procedure. She then accompanies Finn and Jermaine on their quest to find the space telemetry capsule to get Jake back. Along the way, They discover past Betty who was about to go on a trip, but gave it all up for Simon. Past Betty then decides to team up with present Betty to help fix Simon, present Betty refuses then changes past Betty's tickets from next week to today, changing the past so that she leaves and never meets Simon. In the end, King Man reveals that they were inside the telemetry capsule all along and that Betty learned to let go of others. Betty then denies that statement and states that she learned that she needs to try harder to get those things back. She then comes up with a plan to save Simon and to help King Man save Margles from GOLB.
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 daring, as she went to face off against the creature Bella Noche by herself in an attempt to keep Simon from dying.
Like Simon, she appears to be quite educated in magical artifacts and was confident she could return Simon to normal.
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 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 studies, such as science or medicine.
- Simon referred to Betty as his princess while he still possessed some of his sanity.
- Betty's last name is an homage to Stanislav Grof, a psychiatrist who's one of the founders of the field of transpersonal psychology.
- Betty appears to be left-handed since she used her left hand to punch Bella Noche, though she could be Ambidextrous.
- She is now the second human inhabitant of Ooo.
- Betty was originally voiced by Lena Dunham, but is now voiced by Felicia Day.