>>433504382Season 4, the episode is called "I remember you"
There's a wall of text explaining everything, caution, contains spoilers:
WARNING, FUCKING SPOILERS, BITCH:
Betty is Ice King's ex-fiance from before the mushroom war. Ice King used to be a sane man named Simon. (this parts fuzzy for me) While on an expedition to Scandinavia, he bought the crown he now wears, and acquired the Enchiridion from a dockworker.
While trying to impress Betty with his discovery, he puts the crown on and begins to have visions. He tells betty that everything will burn (the mushroom war) and that even she will die. He mumbles about how he can save them all by freezing them, but he won't be able to do it while having to watch betty suffer.
She pulls the crown off his head and he snaps out of it. But she is too scared and she tells him she never wants to see him again.
After that instant, Simon becomes haunted by visions from he crown, to the point where he doesn't even need to wear it anymore. He slowly grows a beard, his skin turns blue, and his nose elongates. This transformation occurs over either years.
Eventually, during his travels in the post-apocalyptic world (about 1000 years before the AT universe), he mets Marceline, he gives her the doll she that her boyfriend sold. Simon tells her that he won't remember her and he's losing his mind. So he leaves her.
Over the next couple of centuries, Ice King is somehow always drawn to Marceline, no matter where she goes, yet Ice King cannot remember her as a child.
She is the only princess/queen he does not court, probably because of their first encounter. In the episode (I remember you), Ice king tries to get Marceline to write him a song to get him chicks. This is where most of the above information comes from.