He is the most overprotective and paranoid parent in the ocean. He is one of the biggest worriers, and can't stop giving out safety tips. He is dedicated to bringing up his son Nemo, and also to being there for Dory. Deep down, he is incredibly brave.
Marlin is not good at telling jokes without taking it too seriously and tends to see the downside of any situation. Luckily, Nemo and Dory always put a smile on his face.
"He's a worrier not a warrior. Meet Marlin, the most doting dad in the ocean. He was a happy-go-lucky fish until a barracuda attacked his wife, Coral. Now he's the most hung-up fish in the sea. He won't let his son leave the anemone, cross a current, or take a school trip without a safety lecture. He has promised to make sure that nothing ever happens to his dear Nemo. But that's the whole problem. He won't let his son live a normal live. It's only when a human diver catches his precious boy, that Marlin learns just how capable he and his son really are."
Marlin and his wife Coral look at the ocean view out of their anemone. They 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 perished after the barracuda attack, 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, 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, Dory meets Nemo, and after remembering Marlin's goal, returns Nemo to Marlin, who becomes overjoyed upon reuniting with his son. However, their heartfelt reunion is cut short when Dory is caught in a fishing net along with thousands of other fish. Marlin, out of fear of losing him again, is initially reluctant to let Nemo go with his plan to save Dory, but upon seeing the determination in Nemo, allows him to do so. Marlin then orders the captured fish to swim down, and eventually, his message is relayed through the fish, who then pull down on the net until the net breaks, freeing them along with Dory and Nemo. Nemo tells his father he doesn't hate him, to which Marlin apologizes to him for his overprotectiveness.
At the end of the film, Marlin and Nemo express their mutual love for each other. After letting go of his son, allowing him to go have an adventure, Marlin, with Dory by his side, proudly watches as Nemo goes off to school.
In the sequel Finding Dory, Marlin and Nemo accompany 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 go on a quest to find her.
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 attack his children and his wife, 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 clownfishes 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.
- In Monsters, Inc., a painting of Marlin can be seen on the wall behind the Chef.
- As pointed out by Nigel, a marlin is actually a type of billfish (such as the sailfish and swordfish) that is popularized by sports fishing.
- In reality, clownfish are pretentious hermaphrodites, 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.