|The following information is from the Adventure Time comic book series.|
It may not be part of the same canon or continuity as the animated series.
Gata is a character in the Adventure time Comics. In an alternate reality, she is Finn and Jake's sister.
Gata is a teenage girl from an unknown species resembling cephalopods. Primarily humanoid, she has periwinkle skin, freckles, a pair of fins on either side of her face, as well as numerous tentacles pulled back and styled as if it was hair.
Gata's overall rather cheerful and cool headed, comfortable and friendly with most of the citizens of Ooo. She's incredibly skilled with machines and technology, proficient enough to give BMO a Giant Fighting Robot form, as well as the Nuclear Power station necessary to power them. She also holds both first and second place on BMO's Scoreboard, suggesting a skill with Video Games as well. Gata also seems to have a habit of giving people over-complicated nicknames, whether this is with everyone, or simply Marceline, we unfortunately don't know.
Gata first appeared after Finn and Jake had unintentionally broken a statue at the bottom of a Lake, after receiving holo-instructions from Joshua to protect it. Once they return back to the Tree Home, Gata is back in the home playing BMO, to the surprise of Finn and Jake, because until that point they had no memory of her at all; despite the fact that there was numerous signs she had been living with them for a long time. She knew Marceline and Princess Bubblegum. She also seemed to know plenty about Finn and Jake, remembering Finn's previous relationship with Flame Princess, and cracked jokes about it. And yet despite all of this evidence, Finn and Jake have absolutely no recollection of her existence. Deciding that perhaps they were afflicted by some sort of a Memory Curse, Gata tries taking them back to their first home in the hopes of triggering some memories of her existence. During this time, they bonded a little, and Finn nearly developed a crush, before she promptly squashed the idea; given the fact that she grew up with him like a brother, and thought the idea gross.
Once the trio had found their way to the basement, they uncovered strange reflecting caves filled with a blueish goop, that seemed to be able to show gateways into alternate universes. One of which detailing Gata's past, and the timeline Finn and Jake were now in.
Gata was originally adopted by Joshua and Margaret back when the family lived in the ancient remains of a large City, presumably dating back to before the Great Mushroom War. Once adopted by the family, Joshua and Margaret were now constantly attacked by thought demons; brain-like demons that can read people's thoughts. While investigating both the origins of the Thought Demons, as well as the mysterious underground caverns beneath their home, Joshua discovered that the Thought Demons have been coming out of Gata's mouth while she slept. Having realized that there would be no way to protect his family if Gata was going to continue making Thought Demons, he used the blue goop within the caves below and removed Gata from their universe, leaving a stone statue shell in her place, and wiping her memory from all existence. At least, until Finn and Jake broke the statue, bringing them to the universe where she had never left.
Now guilt ridden over her existence, Gata believed that Joshua had made the right call in banishing her, to which Finn and Jake strongly disagree. They offer to go into her mouth when she's sleeping, and not only investigate, but ultimately stop whatever's been sending the Thought Demons through Gata. After they entered the Thought Demon Dimension inside of her body, a noise startles Gata, and wakes her up; ultimately sealing the two inside until she can fall back asleep. It is there that they meet an elderly woman resembling Gata's species, who saves them from a horde of Thought Demons, and tells them the tale of Gata's being.
Gata was originally born from a Demon Queen, created before the Great Leaders of Ooo banished the Queen to a prison dimension full of her thought demons, leaving Gata behind as a gate back into the physical realm of Ooo. Whenever Gata would go to sleep, a portal to the dimension would form in her mouth, which vanishes upon her waking up. Given the unexpected size of the prison dimension, the Demon Queen could never find the portal before Gata woke; leaving the queen to scour her dimension for years with no luck. Once Gata fell back asleep, the portal opens next to Finn and Jake, and the old woman reveals herself as the Demon Queen, escaping through Gata's mouth and wrecks havoc onto Ooo, before Finn and Jake manage to cram her back into Gata's body dimension. But she knows it's a temporary solution. The Demon Queen will escape again, and nothing in Ooo seems to be able to kill the Queen. Desperate to save her family from herself, Gata flees to the basement and chooses to sacrifice herself, and seal the Queen away alongside Gata's existence. Finn and Jake rush after her, only to forget what they were doing partway through, and decide to go home; leaving Gata's statue in the cavern untouched, and hasn't been seen since.