Marceline is stated to be a half-demon, implying her mother was human, or possibly something else. Hunson having a child with a non-demon may have caused some great controversy that led to a war. It has been confirmed ◊ that Marceline is half-human. And appears to have been born shortly before the Mushroom War.
In a memory which takes place prior to the Mushroom War, young Marceline and her mother are seen living normally in a trailer home. The succeeding flashbacks all take place in the aftermath of the war. In a destroyed city, Simon records a VHS message for Marceline before parting ways with her.
Marceline has commented several times that she is "over a thousand years old," which makes one think, based upon "Memory of a Memory," that the Mushroom War occurred around a thousand years ago.
He is originally believed to be the only human left in Ooo after the "Great Mushroom War", a mysterious conflict that occurred 1,000 years prior to the start of the series. It is later shown that he was born on Hub Island, part of an isolated archipelago populated by humans.
This was at least 1000 years ago. The only conclusion is that after the first apocalypse, which was probably the Great Mushroom War, humanity rebuilt and was decimated a SECOND time, which may have been some cataclysmic event involving the Ice King.
The Untold Story of the Great Mushroom War (Adventure Time)
How old would Marceline be in human years?
Marceline is a half-demon and half-human vampire. She assumed the title "Vampire Queen" after having killed the previous Vampire King prior to the start of the show. Although she is over 1,000 years old, she takes on the physical appearance of an 18- to 21-year-old.
Shermy is confirmed to be a reincarnation of Finn, which makes sense considering their similarities and the fact that Finn can resurrect when he wants and be another living being due to being an incarnation of the Blue Catalyst Comet.
Later, the reason Martin abandoned Finn is revealed in a flashback in the Islands miniseries where he and baby Finn are chased off from an island as a result of Martin's past con-artist ways. To protect Finn, Martin defeated the guardian that thought they were trying to escape the island on a raft.
Unlike traditional Western vampires, Marceline subsists not off blood, but rather the color red (including "low-grade" shades such as pink). Since she is only sated by the color itself, blood is appealing simply due to its hue (although Ward noted on his Formspring that she "drinks blood sometimes").
Since she never became a vampire, Marceline aged over the past 1,000 years (much more slowly than humans due to the fact that she is half demon). She is ultimately killed when the mushroom bomb goes off and she falls within its liquid, her final act being an attempt to save Finn from the Lich-possessed Jake.
Princess Bubblegum (Real Name: Princess Bonnibel Victoria Bubblegum) is a main character of Adventure Time. She made her first appearance in the animated short "Adventure Time!." She is the ruler of the Candy Kingdom.
Finn was, for a long time, the only confirmed human in the Land of Ooo, to the point where he had an epithet, "The Human", and was referred to by many as "Finn the Human". The reason for this is revealed in the episode "Her Parents," as Lady Rainicorn's dad believed that humans were extinct.
After The Lich was freed by the hypnotized snail he controlled, he killed Billy in vengeance and wore his body like a costume, using the corpse of the hero to manipulate Finn and Jake to get to The Time Room. Finn and Jake still find the fact that Billy is dead something hard to accept.
Billy reveals that they must unite the gems of all the crowns of power, place them in the Enchiridion, and open a portal to the multiverse. However, it is revealed that the Lich is actually disguised as Billy. Finn and Jake then follow him through the multiverse.
Sweet Pig-Trunks, usually called Sweet P for short, is the "rebooted" form of the Lich who was created in "Escape From the Citadel", After being splashed with the Citadel Guardian's healing blood, the Lich grew fresh organs and skin, causing him to take the form of a giant baby with almost no recollection of his ...
Jake turns into his blue alien form seen in "Skyhooks II" and "Abstract". He also briefly turns into Shoko's tiger, thereby revealing that it is one of his past lives.
Beth is very clearly a descendant of Jake the Dog and the pups he had with Lady Rainicorn (a rainbow/unicorn species that comes from the crystal dimension). Meanwhile, Shermy is a cat-like creature who some fans believe to be the reincarnation of Finn, but that's just a theory.
Finn is eager to help until Slime Princess says that she needs Finn to marry her or else she will lose the Slime Kingdom. Finn states that he is unable to do so, and Ice King interrupts and volunteers himself as husband before Jake tells him off.
His current biological age is 34, as stated by Steve Wolfhard in the Conversation Parade podcast, meaning that at some point his aging slowed down from when he was a puppy. Jake says in "Daddy-Daughter Card Wars" that he's lost track of how old he is, but thinks he might be almost 40.
Something during the post-apocalyptic time, Elise fell gravely ill, and hid her illness from Marceline. She would later succumb to her illness, and tried to shield Marceline from her death by convincing her to leave her behind and find the location of a safe shelter without her.
