- “Daddy's got you. I promise, I will never let anything happen to you, Nemo.”
- ―Marlin comforting the egg of his last surviving child
Marlin is a clownfish who makes his home inside a sea anemone in the Great Barrier Reef. He once had a mate Coral who died in a barracuda attack, along with almost all of his children. Also, as a result of the attack, he has one son, Nemo.
Marlin can be somewhat overprotective of Nemo. He also tends to be neurotic, worrisome and pessimistic, believing Nemo can't swim well when he actually can swim pretty well for a fish with a small pectoral fin. However, this does not stop him from going on a journey to find his lost son, proving his determination.
Marlin was initially a very playful, happy, and easygoing clownfish as shown through the interaction with his wife. He was shown to be slightly lazy, as he wanted to name half of his children Marlin Jr. and the other half of his children Coral Jr. However, he was also shown to be very brave, as he attempted to defend his wife against the barracuda who sought to eat his children and his mate, but was ultimately repelled and knocked unconscious and into the sea anemone before he could do anything. This protected him, but also cost him his entire family, with the exception of Nemo, who was left with a crack in his egg, which could have caused his "lucky fin".
From this moment on, Marlin became overprotective, neurotic, paranoid, worrisome, and pessimistic, as he made several practices and conditions for Nemo and himself to follow to make sure they were protected and safe. He became paranoid and neurotic about any potential danger that could occur to Nemo and as such rarely left Nemo alone due to the traumatic experience of losing the rest of his family to a barracuda attack, and not wanting the same thing to happen to Nemo. However, this overprotectiveness is what put somewhat of a strain between him and Nemo, as Nemo felt suffocated and oppressed by it.
He also seems to, initially, be very serious, as shown when he couldn't tell a proper joke to some of Nemo's classmates' parents because he kept explaining obvious aspects of the joke, causing him to be considered unfunny despite the apparent notion that all clownfish were supposed to be funny, which became a recurring gag throughout the movie as others state, "He's not very funny for a clownfish".
During his journey, Marlin was on the verge of losing Nemo, the only family he had left, and was rushing as fast as he could to save Nemo. This caused him to have little patience for Dory's antics and to other sidetracks.
However, despite these qualities, Marlin has proven to love his son more than anything and anyone in this world, and it is this love that fuels his determination, strength, courage, and bravery as he journeyed throughout the sea to find his son after he is kidnapped by humans. Marlin overcame with the bravery and courage not seen in his previous behavior to recover the person he loved most in the world despite the odds being against him.
Throughout the journey, he makes several friends and begins to understand that he should have given Nemo more freedom, particularly through his interactions with Dory and Crush because, as Dory states, "You can't let nothing happen to him, because then nothing will happen to him", meaning if Marlin doesn't let Nemo grow up and get out in the world, he won't be able to live life to the fullest.
He also begins to enjoy himself throughout his journey, and slowly sheds his serious personality, as shown when he had fun when he competed with Dory in racing to see who could get out of the jellyfish valley first and finally managed to have fun with his son Nemo by the end of the film.
At the beginning of the film, Marlin and his mate, Coral are expecting 400 children, which are due to hatch soon. The two are discussing names for their children, when a barracuda attacks. While Coral disappeared, and most of the eggs perish, Marlin and one slightly damaged egg survive. Marlin chooses to name his remaining child Nemo, a name Coral had liked.
As Nemo grows, Marlin becomes very overprotective of him, in part due to Nemo's unusually small right fin, which was a result of the earlier attack. One day, Marlin reluctantly sends Nemo to school. At one point, thanks to the alert of one student, Kathy, Nemo swims out into the open ocean to touch a boat as a result of a dare, much to the horror of Marlin. Nemo is captured by a diver. However, the diver drops a mask which bears his address, though Marlin can't read it.
Marlin is able to obtain the help of Dory, who is able to read the address but has short term memory loss, and learns that Nemo has been taken to Sydney, Australia. Marlin, along with Dory, sets out for Sydney in an attempt to rescue his son. At one point on their journey, Dory is injured by jellyfish. Marlin swims through the jellyfish and is able to rescue her. (He explains that since he lives in an anemone, he is resistant to stings.) Marlin soon passes out, and awakens on the back of a sea turtle named Crush, who is riding the East Australian Current (EAC). After Marlin shares his story with some young sea turtles, word of mouth soon reaches Sydney and eventually, Nemo.
Marlin eventually reaches Sydney's harbor by way of a whale, after becoming lost in the polluted water. Nigel, a pelican who has heard of Marlin, and knows Nemo, takes Marlin and Dory to a dentist's office where Nemo was taken. There, they find Nemo, apparently dead (Nemo was actually faking it in an attempt to escape). Saddened, Marlin leaves for home, leaving Dory behind despite her claims that her memory is better with him. Later, while swimming through a fishing area, he hears a familiar voice calling out to him. He soon realizes that it's Dory and a still alive Nemo. Dory had met Nemo and soon remembered everything, leading him to Marlin. Marlin happily reunites with his son, but the moment is cut short when Dory is caught in a fishing net. Marlin watches as Nemo is able to successfully rescue her with skills he learned while in the tank at the dentist's office. Marlin had disobeyed himself and becomes less overprotective. The end of the film shows Marlin, along with Dory, waving goodbye as Nemo goes off to school.
In the sequel, Marlin, along with Nemo, accompanies Dory to the Jewel of Morro Bay, California, in her journey to find her parents, whom she suddenly remembers. When Dory gets kidnapped by the volunteers of the Marine Life Institute, the father-son clownfish duo goes on a quest to find her.
Marlin features in both the Finding Nemo Submarine Voyage at Disneyland, which features Marlin searching for Nemo, who has become lost once again. He is also in The Seas with Nemo & Friends at Epcot, which features a similar storyline. He also appears in Finding Nemo: The Musical as an articulated puppet, which is used to retell the film's story in the style of a Broadway play. Duplicates of the puppet were also used in the Pixar Play Parade, and the Paint the Night Electrical Parade. Marlin also appears with Dory in Nigel's mouth in the Mickey's Storybook Express parade.
- In Monsters, Inc., a painting of Marlin can be seen on the wall behind the Sushi Chef.
- As pointed out by Nigel, a marlin is actually a type of billfish (such as the sailfish and swordfish) that is popularized by sport fishing.
- In reality, clownfish are protandrous hermaphrodites (meaning they are born male but can change sex later in life), so whenever a female clownfish within a school of fish dies, a male will change gender to make up for the death and mate with the other male.
- Scientifically speaking, if Finding Nemo had been more accurate to nature, Marlin would have become female shortly after his wife's death. This would have made him Nemo's mother, not his father.
- Common or Ocellaris Clownfish are considered good beginner fish for salt water aquarists.