Finn the Human, later revealed to be Finn Mertens, is the protagonist of the cartoon show Adventure Time. His title "the Human" comes from the fact that in the post-apocalyptic setting of the show, he is the last true human alive, although he shares the world with a variety of sentient creatures. Although it has not been stated in the show, the name "Finn" probably comes from the Irish hero of the same name, who shares a few features with Finn the Human. In the pilot of the show, Finn's voice actor was Zack Shada, but when the show entered production, the role went to Zack's brother, Jeremy Shada. Jonathan Frakes has also voiced alternative versions of Finn. For the first few seasons of the show, Finn appeared in every episode, although sometimes not as the main character. Starting in the most recent seasons, he has been absent from some episodes, although he is still the main protagonist of the show.
Finn has a distinctive character design, with a rabbit hat and a backpack. In the series pilot, he is said to be 12 years old, and he has aged two or three years over the course of the show. He lives in a tree house with his friend and adopted brother, Jake the Dog, and serves as a champion for Princess Bubblegum, ruler of the Candy Kingdom.
When he is first introduced, Finn is cheerful and focused on external goals: he fights monsters, saves people, and spends time with his friends. He isn't very introspective, and has no interest in romance. He believes he can solve every problem, and never looks at failure as a possibility.
Over the course of the series, starting around the third season, he undergoes much character development. He is confronted with problems, both in adventuring and in relationships, that he can not easily fix. His optimistic nature and reckless actions, which were at first played for laughs, become more dramatic. His family background, at first only alluded to, becomes a major plot point as Finn finds his biological father, with complicated consequences. There have also been several alternative versions of Finn depicted, either past lives or alternative timelines, and these often showcase Finn as being a tragic or wounded character: one of the recurring characteristics of his alternative selves is that they are missing an arm. None of this character development is done overly dramatically: while Finn is developed, it doesn't overcome the generally light, fast moving tone of the show.
Like much on Adventure Time, there is not much about Finn that sounds that interesting when described: he is an adventurer with a silly hat. The appeal of the show is how much it can draw out of its basic premise.