Petribell

Petribell is the het ship between Fionna Campbell and Simon Petrikov from the Adventure Time fandom.

Canon
Prismo, wanting to have something for himself, decided to create a world. It ended up looking like Finn and Jake's universe, but he didn't care. Unfortunetly, it was illegal for him to make that world without a wish, so he decides to hide it inside Ice King's "domain". Oblivious to what that meant, Ice King accepts ("especially if there's a hot lady in it") and that's how Fionna-world ended up inside Simon's brain.

Adventure Time
Ice King writes a fanfic based on his weird dreams, not exactly knowing where those come from. (Thought he does know he receives them from a red laser.) He has a soft spot for Fionna and secretly wishes her and Cake were real.

In Come Along With Me, he tries to connect to Betty as Simon, using the song Oh, Fionna. But he messes up by pronouncing Fionna's name, which leads to Betty getting angry at him for trying to stop her and Maja the Sky Witch exploding sending the two into GOLB' s mouth.

Fionna & Cake
In the Fionna & Cake miniseries, everyone in Fionna-world have the memories of their previous magical lives erased. Twelve years have happened, but for three days in-a-row, our two titular characters have dreams about an Ice Prince, to whom Fionna is smitten with. After she takes Cake with her on her tour bus, she askes the people in it if they know about dream theory and explains her reason to ask. Marshall tells her that "[she] only dreams about a cool Ice Prince because [she] got dumped by a hot DJ Flame", to which Fionna gets flustered. Later that day, Cake runs away to a portal, and Fionna tries to get back to her. The next morning, she finds another portal inside a fridge in the bakery and gets sucked in.

Meanwhile, Simon has find a terrible job and a little girl who keeps bugging him about Fionna & Cake for three days. He tries to forget about his bad day at Dirt Beer Guy's bar, but the latter, Finn and TV reminds him about the fanfic/book series. He gets forced into an adventure by Finn. Despite Finn's best intentions, he feels worse. He calls Marceline, like Finn suggests him to, but she is busy trying to get matching tattoos with her girlfriend P.B. He finally decides to read Fionna & Cake and it gives him determination. This newfound feeling is what drives him to open a portal to GOLBetty. But he is once again reminded of Fionna and Cake, and so Cake arrives in Ooo. He tries again, but and this time it's Fionna who gets out of his head.

Upon seeing someone down, Fionna checks on Simon and mistakes him for the Ice Prince.

Fanon
Some fans have started to ship them since Episode 1 of Fionna & Cake and some after Episode 6. Despite making sense as they are narrative foils to each other and having a good chemistry, many others fans still see Fionna as a kid (even though she's actually 29 when the two actually meet in person) or as Simon's kid (though it's purely interpretation on their part). So the ship is considered controversial because of it. There are fans who like both possibility (not at the same time obviously) because of how sweet their friendship is.

Fandom

 * FAN FICTION