Finn the Human

Finn Mertens (also called Finn the Human, Pen in the original short, and identified as P-G-8-7 Mertens) is one of the main characters in Adventure Time

Background
It is revealed in "Min and Marty"  that Finn was born on the Hub Island (one of the human-inhabited islands outside the land of Ooo) to his father Martin Mertins and his mother, Minerva Campbell. Having been a known con artist, Martin is confronted by an old foe and forced to flee the house with baby Finn while Minerva was at work. Martin brings Finn onto a raft, only to be attacked by the Guardian. Finn and Martin are separated as a result, with Finn being sent adrift towards Ooo and Martin disappearing.

As shown in "Memories of Boom Boom Mountain," the infant Finn eventually made it to Ooo, where he ended up alone in a forest. There, he went "boom boom" (Finn's word for defecation) on a large leaf and fell on it. He lay there crying until Jake's parents, Joshua and Margaret, rescued him from the wilderness and brought him to their home. They raised him alongside Jake, who, along with Jermaine, became Finn's adopted brother.

At end of the Distant Lands episode "BMO," Finn and Jake are shown at younger ages than their first appearances on Adventure Time, stick battling in front of the Tree Fort. By this point in time, Joshua and Margaret have most likely passed away, and Finn and Jake presumably no longer live with Jermaine.

It is revealed in "Susan Strong," that Finn knows nothing of his human parents or any other humans. In "Henchman," Finn states that Jake told him that he originated from a cabbage as a joke. However, Finn later learns about his biological father, Martin, in the episode "Billy's Bucket List" and his biological mother, Minerva, in the episode "Min and Marty."

Status as the "last human"
Finn was, for a long time, the only confirmed human in the Land of Ooo. The reason for this is revealed in the episode "Her Parents," as Lady Rainicorn's dad believed that humans were extinct.

Up until "Susan Strong," Finn believes he has never met another human and becomes morose and "soul-searchy" when he thinks about it. However, Finn encounters Susan Strong, a member of a tribe of mutants called Hyoomans. However, in "Beautopia," it is hinted that Susan Strong is unlike the rest of her tribe and may potentially be another human. In "Dark Purple," Susan removes her hat and reveals she is a human with a device on her head, making her possibly some sort of cyborg. However, it is unknown if Finn knew about this.

As of "Betty" Finn is no longer the last confirmed human because Betty Grof traveled through time from before the Mushroom War.

In "Billy's Bucket List," Finn is told by a deceased Billy that his father, Martin, is still alive and is trapped in the Crystal Citadel. This prompts Finn to travel to free his father in the episode "Escape from the Citadel," making him one of four known humans in existence at the time.

In "Preboot" it is revealed by the "human plus" Dr. Gross that are more humans outside of Ooo. This is further explained in the miniseries Islands, where it is revealed that roughly a thousand years prior to the main events of the series, a group of surviving humans from the Mushroom War fled Ooo on a container ship (as seen in Stakes) to escape from the vampires, oozers, and a comet. They eventually settled on a secluded island chain far from the mainland. Over the next thousand years, their community thrived and developed into an advanced technological society. While it was a veritable utopia, there were those few who occasionally grew dissatisfied with their rigidly structured lives and attempted to flee the island; these "hiders" were, in turn, hunted down and returned by specially trained "seekers." Finn, Jake, Susan Strong, and BMO went to the islands so that Finn could find out about his past and his species.

Finn for the first time saw large amounts of humans. Finn met his mother, Minerva while on the island. His mother is no longer a human and is now a virtual consciousness (A.I.) due to sacrificing her body in order to treat the human race from a virus. Finn was offered to live on the island but Finn decided to go home to Ooo rather than live with the rest of his species. So after the miniseries Islands, Finn being "the last human" was completely dispelled. Though in the Islands CN Side, Adam Muto stated that Finn being the last human in the world was never the most important thing about him.

Although various other humanoids exist in the Land of Ooo, most have been confirmed by the show's creator, Pendleton Ward, to be mutants, such as Penny, Phil, or the Old Man Henchman. According to Pendleton Ward's FAQ page, even Finn is mutated in some way.