Finn Mertens (simply known as Finn and formerly known as Pen in the original short) is the main protagonist of the Cartoon Network series Adventure Time, as well as one of the 4 main protagonists of Adventure Sponge.
He was voiced by Jeremy Shada, who also voice as Lance from Voltron: Legendary Defender and Cody Maverick in Surf's Up: Wavemania.
Finn is a 5-foot tall teenage human with fair skin, blue eyes, and blonde hair, which varies in length due to the occasional haircuts. The most prominent piece of Finn's attire is his trademark white hat with two ears sticking out on top that covers his entire head minus his face. The rest of his outfit consists of a munsell blue t-shirt, darker blue denim shorts, rolled-down white socks, and a pair of black shoes. He also wears a two-colored lime green circular backpack.
Though at times violent and aggressive, Finn is a brave and righteous hero. He has a very strong sense of responsibility and gets upset when he is unable to help others. He is almost physically incapable of doing anything evil or unjust, and will struggle greatly against doing such things, as seen in several episodes such as "City of Thieves," "The Eyes," and "The Enchiridion!" Even though he acts like a normal boy, Finn acts like a gentleman when it comes to the ladies.
Finn is shown to be a fearless thrill-seeker and enjoys putting himself in the midst of danger simply because he enjoys a challenge. However, as illustrated in "Ocean of Fear," Finn has an unexplained phobia of the ocean, or thalassophobia, which may be explained in a future episode, despite the fact that he can effortlessly come into contact with any other body of water. While he portrays himself as a rough and tough hero, Finn has a tender side capable of love and compassion, as shown by his boyhood crush on Princess Bubblegum and his brotherly relationship with Jake. However, Finn seems to hate romance movies and kissing because in "Go With Me," he actually vomits during a kissing scene in a movie. But at the end of "Dream of Love," Finn was not grossed out by Tree trunks and the pig kissing.
Finn’s aspirations to be a great hero makes him somewhat of a moral sheriff in the Land of Ooo, as he is willing to help and protect any innocent person or creature. Finn appears to be somewhat simple-minded, seeing that he gets a lot of conflicting emotions when it's unclear whether something is good or evil (exemplified by his dealings with Marceline in the episode "Henchman").
Many aspects of Finn's personality are shrouded in mystery. In the episode "Rainy Day Daydream," it was shown that Finn believes "imagination is for turbo nerds who can't handle how kick butt reality is," showing Finn not only accuses imagination users as turbonerds and believes life is fantastic. Finn then refused to use his imagination until he had to reactivate Jake's "bombastic personality" in his imagination land, (which was just a basic gray and white empty plane hinting that Finn isn't very imaginative but that was contradicted in "Power Animal" when the gnomes harnessed the energy from Finn's imagination) which was inhabited with a few dainty creatures such as Bellamy Bug who represented Finn's secret desire to be civilized (that he will not admit to). Finn's imagination was powerful enough to affect the real world (like Jake's but to a lower degree) and turn on Jake's imagination.
Finn can occasionally be a little self-centered such as in "Freak City." Finn can also be very stubborn, such as in "Memories of Boom Boom Mountain," "Rainy Day Daydream," and especially in "Another Way." Finn can also be easily tricked, as in "The Eyes" when Jake convinced him that the giant hawks would take the horse to a land of sunshine and love stuff and take care of him. He can get overly angry and yell as in "Trouble in Lumpy Space" and "Tree Trunks," but he immediately feels guilt and remorse and apologizes when he can. Finn prefers to do work and not take the easy way out ("Business Time").
Before the School Daze series, Finn teamed up with the boys of South Park to compete against Scott Tenorman in a swim race in order to get Patrick out of middle school. They succeeded, however, Scott proceeded to take Patrick hostage and threatened to kill him. That’s when Jesus showed up and took him to hell. After that, He made a few minor appearances in the series.
In “Tom’s Birthday”, he attended Tom’s birthday party.
In “Michael the Explorer”, he, along with many of his friends, was taken hostage by Trent and was forced to watch Dora the Explorer until Michael’s group rescued them. In “The School Daze Thanksgiving Movie”, he joined Tom and co. on a mission to rescue Alex & Caillou, who were kidnapped by the Prospector.
In “Uncle Funtime: Thanksgiving Edition”, he attended the Thanksgiving celebration with Tom and co. at Gammy’s house. In “School Daze 2: A Jolly Good Battle”, he was seen watching the Regular Show Finale.
In “Gathering the Aquatic Heroes”, he joined Tom and the gang as they were going to An Unlikely Story, where they read a few books and listened to people sing. During the Uncle Funtime 4 Arc, he joined the gang on a trip to New York City, as well as rescuing Patrick from Team Rocket afterwards.
In “Holidays with Spongebob & Rigby”, he celebrated Halloween and went Trick-or-Treating with Tom and the gang. In “Pain, Pain, Go Away”, he helped Caillou’s group by taking them to the Land of Ooo so they can find an antidote for Stewie’s Illuminati Tumor..
In “Ronny’s Not A Fan”, he, along with Spongebob and Patrick, went to Westwood to warn Michael and Uncle Jerry that Ronny was planning to kill them. They then stayed with the gang for the rest of the day, as Ronny was chewing Michael out for neglecting him before the two eventually reconciled.
In “Paddle to the Green”, he, along with Jake, had a present prepared for Spongebob’s birthday party, but unfortunately, it was stolen by Mr. Greenie, so they have to team up with Tom and co. to get ti back. After a chase on the water, they got the gift back and gave it to Spongebob.