Marlin is a clownfish who makes his home inside a sea anemone in the Great Barrier Reef. He once had a mate, named Coral, who died in a barracuda attack, along with all but one of his children, whom he names Nemo.
Marlin can be somewhat overprotective of Nemo, although he means well. 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 due to egg damage from the barracuda attack. 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 seen through the interaction with his wife, Coral. 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 (assumed dead) 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 resulted him in having a tiny right 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 on his relationship with his son, 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 these obstacles with 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 that 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.
Marlin first appears in the flashback during the events of the first film where Nemo gotten kidnapped by the divers and was worried that he lost his son after a boat (which is Philip Sherman's boat that Nemo touched from Finding Nemo) zips above Dory's head. Soon as Dory swims away, she collides with Marlin as the adventure of Finding Nemo begins.
One year after Nemo's rescue, Dory is the next-door neighbor to Marlin and Nemo, living in a purple coral bowl, similar to her original home in the Marine Life Institute. Marlin and Nemo are disturbed by Dory in the middle of the night who tells her to go back to bed until he gives up and gets ready for the day. The next day, Marlin, Nemo, and Dory go on a field trip to Nemo's class (much to Mr. Ray's annoyance) to a stingray migration. After Dory is knocked unconscious by a large flock of migrating stingrays and Dory decides to swim to the drop off to find her parents after mumbling the "Jewel of Morro Bay, California", Marlin refuses to go at first until Dory asks him if he's ever missed his family, which brings up memories of Nemo's capture. However, he begrudgingly takes Dory and his son with him as they go to see Crush and ride the current to California. In it, Marlin, Dory, and Nemo hang onto Crush as the three arrive into their destination in sunken wreckage in Morro Bay, California where he and Squirt wish the three good luck into helping Dory find her missing parents.
Upon arriving in Morro Bay, he, Nemo, and Dory arrive in wreckage where hermit crabs keep shushing the three as they swim along and the three are chased by an enormous giant squid after seeing its eye and fearsome beak inside a crate. While trying to escape, Dory and Marlin are worried that the giant squid grabbed Nemo with one of its tentacles and tries to eat him with its fearsome beak as the two escape from the marine predator after the squid is stuck on a sunken crate while chasing the three. The squid is suddenly crushed as the three suddenly escape the sunken wreckage and arrive in the kelp forest in Morro Bay, California where Marlin tells Dory to slow down because the giant squid is crushed when it tried to eat Nemo.
Upon stopping in the kelp forest of Morro Bay, California, Marlin is worried that Nemo is injured after being chased by the giant squid in which he shoos Dory away after heavily expressing Nemo who is injured during the chase, leaving her insulted greatly and causing her to swim off in despondency. Upon arriving in the surface of the Marine Life Institute, Dory gets kidnapped by two volunteers of the Marine Life Institute, as the father-son clownfish duo goes on a quest to find her. After Dory is captured by a group of MLI rescue volunteers, Marlin is worried about himself that he should have never insulted Dory after escaping from the giant squid, upset that Dory is captured by volunteers of the Marine Life Institute.
Upon hearing two sea lions telling him to be quiet, Nemo suddenly attempts to ask but Marlin warns him that he should not talk to the sea lions, explaining to him that the two are natural predators that could pounce on them at any second. Nemo then explains to his father that the two sea lions do not have the chance to pounce at them while Marlin asks the two about the Institute in which the two explain that it is a fish hospital. Fluke and Rudder help the two clownfish get inside the Institute by calling a flock of loons, including their friend Becky. After Fluke introduces their friend to Marlin and Nemo, she begins to peck at Marlin; Fluke tells the loon to bring Marlin and Nemo to Quarantine, and Marlin is suggested to imprint by looking her in the eye and do a loon call.
After Gerald brings a green bucket to Fluke and Rudder, Marlin suggests that he and Nemo need water as the two sea lions kick and shoo him off from their rock after inviting them and giving the bucket to them on their rock. Meanwhile, Marlin and Nemo board on the green bucket carried by Becky as she flies to help take Marlin and Nemo get to the Institute while the two sea lions wish them good luck into their quest to find Dory and help her find her parents. As the two clownfish are carried by Becky, Marlin is disappointed that Becky is confused and doesn't know where to find the quarantine area and accidentally leaves Marlin and Nemo on a branch, thus she starts eating popcorn that fell into a floor after Hank bumped into a girl who has a cup of popcorn while trying to get to the Open Ocean exhibit. The father-son clownfish duo, still in the green bucket, plan to approach Becky so that she could pick up the bucket again; however, the branch flings the two clownfish as they end up in a small tank outside a gift shop.
Upon ending up in the tank, Marlin and Nemo see Becky carrying the green bucket to the quarantine area's roof as Marlin tries to call Becky for attention to rescue the two, but Becky cannot hear Marlin yelling for help from him. Upon seeing a wind-up toy fish that resembles Dory, behind the two clownfish is a spray pool in which the two clownfish leap out of the tank as they leap on the nozzles launching the two and leading them safely to a tide pool exhibit. Upon arriving, Marlin and Nemo suddenly hear a clam who doesn't know who Dory is as they approach them. The clam explains to Marlin and Nemo that he did not have anyone to talk to for years until Marlin and Nemo ended up in the tide pool exhibit in which Marlin tells the clam that they can't stay for a while as the two need to continue their quest in helping find Dory. The clam tells the two clownfish to stay with him and talk to him for a while before the two can continue their adventure.
Marlin and Nemo later appear swimming through the pipes as the two approach Dory who finally finds her. Marlin explains to Dory that they encountered a talkative clam in the tide pool exhibit in which Nemo explains that he and Marlin backed away from the talkative clam as the two ended up in the pipes. Dory suddenly guides the father-son clownfish duo to the parts where her missing parents would be and they would be excited after she finds them. Marlin then apologizes to Dory for being too harsh by insulting her and causing her to swim in despondency. Soon as Dory continues arriving at the area where she would find her parents, she and the clownfish duo end up in the quarantine area in which the two leap from tank to tank to find her missing parents, but one hits the three as they end up in a mop bucket in which Hank lifts them up in a cup of water and brings them to a tank of blue tang fish. Upon encountering and asking the blue tangs about her parents, some of them explain to her that her parents are not in this area, leaving Nemo, Marlin, and Dory in worry. When Hank is worried that the truck is ready to depart for Cleveland and he brings Dory and then escapes from the MLI employees, Marlin, Nemo, the blue tangs, and the rest of the species of fish are later taken to the truck that leads all the fish to a permanent aquarium in Cleveland, leaving Dory all alone.
Marlin and Nemo are later seen inside the truck and some otters arrive inside with Dory to the rescue as Hank puts her in a tank where Marlin and Nemo are at where the two are happy that they met Dory again. Soon after the otters are shooed away by a truck driver and the traffic begins to move, Marlin suddenly calls for Becky's attention to rescue the two clownfish as Becky suddenly appears perching on Hank's head. Soon as Nemo and Marlin leap into the green bucket, Dory is suddenly abandoned as Becky brings the two to the ocean. Upon arriving back in the ocean, the two clownfish suddenly encounter Destiny and Dory's parents Charlie and Jenny. Marlin suddenly tells the two that Dory is still inside the truck while he tells Becky to return to the truck to rescue Dory. Soon as the truck prepares to leave for Cleveland, Marlin, along with Nemo, Jenny, Charlie, Destiny, and Bailey prepare to stop the truck again in which they suddenly miss the truck. Marlin, Nemo, Destiny, Bailey, and Dory's parents later see the truck hijacked by Dory and Hank being driven into the ocean by falling off the cliff freeing all the fish inside the truck while Dory and Hank fall into the ocean.
During the epilogue, Dory finds Nemo and Marlin, along with her parents and the school children by playing hide and seek as they attend a substitute class by Hank, Bailey, and Destiny. After Dory says goodbye to her parents, Marlin suggests to Dory to return home in which Dory tells her that she would look at the drop off in which Marlin, (who is anxious about her) doesn't think going back to the drop off is a good idea because he thinks Dory could be lost again. Dory tells him that she would enjoy the view safely. Marlin suddenly accompanies Dory looking over the drop off in which he finally relaxes against the drop off, praising her that she did it by finding her parents, sparkling one last flashback when Dory was a baby and her parents proudly praise her for following the trail of shells all the way back home to the coral, as the two allow her to play with the other fish children as she swims off happily. Back to her own mindset, Marlin and Dory are last seen watching the drop off view safely as Dory smiles feeling accomplished and satisfied about herself after finding her missing parents as she deems the memories "unforgettable".
- 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 sex to make up for the dead female and mate with the other males.
- 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.
- Marlin was originally voiced by William H. Macy, who actually had all his lines recorded when Pixar decided to change the voice to actor Albert Brooks.
- People expected Marlin to be the deuteragonist of Finding Dory, but it turns out that he has less screen time than Hank, who is the deuteragonist of this film.