Prior to the start of Adventure Time, Finn was adopted as an infant by two anthropomorphic dogs named Joshua and Margaret, who found him in the woods. His best friend and adoptive brother Jake the Dog ( John DiMaggio) accompanies him on many of his adventures in Ooo, the fictional world that the series is set in. The mystery surrounding Finn's origins and whether he is the last human left in Ooo is a major part of the character's narrative arc. Later in the series, it is revealed that there are other humans still living in Ooo and that Finn's parents Martin Mertens and Minerva Campbell came from a community of survivors living on a chain of islands.

There is a secret GPS implanted on Finn's ear by Princess Bubblegum, as stated in " Lady & Peebles." Finn appears to not care for his hair much whatsoever, showed in how he frequently cuts it completely bald when asked (such as in " To Cut a Woman's Hair"), and in " Davey" when he shaves and slathers his hair in molasses, while BMO cries and claims Finn's "beauty is destroyed" and then says he looks like "the devil!" Finn and Jake gain magical abilities in the episode " Wizard," but they never use those abilities in any other episode. Pendleton Ward later revealed on his Spring that all of the magic that Finn and Jake gained in that episode was contained in their robes, which were destroyed. [8] Likewise, in " The Chamber of Frozen Blades," Finn and Jake gain ice-ninja skills, but it is unlikely they will use them again because, according to Adam Muto, "They didn't have enough time to commit the incantations and hand positions to memory." [9] Finn seems to speak Latin (as evident in " Morituri Te Salutamus,") although this may be caused by the magical powers of Fight King's arena. In " Daddy's Little Monster," for the first time, he becomes an antagonist by wearing the Nightosphere amulet, although it is only temporary. In " The Tower" Finn gains a new mental arm with the ability to interact with the physical world. His emotions seem to cause this, along with the ability of telekinesis. Since it was linked with his emotions, it exploded into a huge arm of spikes when he confronted his "dad," and disappeared when Finn's animosity abated. In " Flute Spell" he was granted permanent immunity to electricity by the Forest Spirit.Finn McCool was an adventurer and hero of Irish mythology. "Finn" is the Anglicized version of "Fionn", meaning "blonde". Finn is shown to have an unrequited love for Princess Bubblegum throughout much of the series. He and Jake frequently have to rescue Princess Bubblegum from the obsessive Ice King. Finn's past life as Shoko, an orphaned thief who befriends and then betrays Princess Bubblegum, influences his present-day infatuation with Bubblegum. Finn's friendship with Bubblegum becomes more distant as the series progresses. Despite being mischievous from time to time, Finn is a very kind-hearted person. It's shown multiple times that he shows clear concern for others, even for those who have willingly hurt him, such as in " The Enchiridion," he returned the dollar to the giant (note that this was the very same giant who ate his beloved brother Jake and even kept insisting that he was dead). Furthermore, he shows clear contempt for anything that is done out of immoral reasons and a consistent drive for righteousness—to "do the right thing." Finn begins dating Flame Princess ( Jessica DiCicco) in the third-season finale "Incendium", after realizing that Princess Bubblegum is too old to be compatible with him. Although Flame Princess' fiery nature makes their relationship difficult, they try to make it work. Finn and Flame Princess eventually break up. Finn's relationships with both Princess Bubblegum and Flame Princess cause him to have to deal with rejection and reconciliation, and he emotionally matures as a result. From Finn’s picture, we can’t see much of his shoes’ detail except its color, so it’s safe to go with something comfortable like slippers.”

Finn was always fun because he started very agro [sic] and that was fun to write for a while, but as he got older it didn't make sense to keep writing him that way. He became a mellower version of himself and more introspective and that was satisfying to follow. Finn the Human's character arc has been highlighted by critics for its honest realistic portrayal of adolescence and universal struggles associated with coming-of-age. Brandon Zachary of Comic Book Resources described Adventure Time as "one of the most expansive fantasy series in modern history, centering around a kind but unassuming boy growing into a noble young man." Writing for Vox, Emily VanDerWerff called the series "this era's finest coming-of-age story", mentioning how Finn grew "from boy to almost-man, an adolescent who's slowly figuring out how to be both a good friend and a good person." Oliver Sava of The A.V. Club praised the episode "Breezy" for its relatable depiction of Finn's developing sexuality and his search for emotional fulfillment. As an older man in his mid to late 20s, Finn appears to be not wearing a shirt anymore due to his tattoo of Jake.

In Issue 9, it is revealed that Finn had been called "Pen" in the original short because of his love for pens. Although originally, he was most likely named after the creator of Adventure Time, Pendleton Ward. A future Finn then takes his red sleeping bag when he and Jake leave to help Lady Rainicorn, also leaving a note of advice telling Finn to use his real name. This, however, is most likely non-canon. As of " Betty", Finn is no longer the last confirmed human because Betty Grof traveled through time from before the Mushroom War. In " Obsidian", he appears in his mid to late 20s as an adult and has a full-grown beard, along with a tattoo of Jake on his chest, implying that his adoptive brother may have died by this time. He also has more battle scratches. Finn is also friends with BMO (pronounced Beemo), a sentient handheld game console who is Finn and Jake's roommate. Finn creates the robot Neptr (Never Ending Pie Throwing Robot) as part of a prank, but NEPTR soon becomes a friend to him. Finn ends up developing a close friendship with Marceline the Vampire Queen ( Olivia Olson), despite her initial role as an antagonist. At the beginning of the series, Finn's voice is more high pitched, but later near the latest episodes, Finn's voice is a bit deeper from puberty.

This contradicts with " Min and Marty," where he is revealed to already have the name 'Finn' before being separated from his parents. In the same episode, it is shown that when Finn drifts alone in the raft, there is also a stuffed bear having his name tag on it along with him.In the early seasons of the series, Finn is enthusiastic and readily resorts to violence to settle conflicts. About this aspect of Finn's character, Ward said that he chose to "add a little wrath" to add depth to the character. By the end of the series, Finn is much more reserved. Adventure Time showrunner Adam Muto stated that this theme is part of the reason why Finn did not have a sword for much of the finale. Muto said that this story arc made Finn one of the most satisfying characters for him to write on the show. In the Adventure Time: Distant Lands special " Obsidian", an older Finn appears. His hair has darkened and is now a darker dirty blonde colour rather than the golden-yellow blonde from when he was younger. In " Lemonhope Part 1," it is shown that Finn has been taking classes at a school run by Princess Bubblegum, one of his classes being geometry.

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. In the episode " The Vault," it's revealed that one of Finn's past lives was a comet. This could have some connection to the Catalyst Comet. This was confirmed in " The Comet," as the Catalyst Comet was, in fact, one of Finn's past lives. Finn appears as a playable icon in Geometry Dash (2.1 Update) You can also color him to look like Finn. Finn and Jake had a balloon depicting them in the Macy's Thanksgiving Day Parade in 2013-2015. The balloon retired after the 2015 Macy's Thanksgiving Day Parade.In Legends of Ooo, it is seen that Finn has a telescope that he uses to spy on Princess Bubblegum and Flame Princess.