Marlin

Marlin is the main protagonist in Finding Nemo. He is Nemo's father, who tries to rescue him in Sydney. He got help from Dory to get there.

Biography
"After losing his wife and family to the ocean, Marlin is left alone to raise his only surviving child, Nemo. He struggles with his inability to let go of Nemo as his son begins school. When Nemo is suddenly taken away by scuba divers, Marlin must try to find in himself the courage, wisdom, and faith needed to search the unpredictable ocean to bring his boy home - the very characteristics he needs to allow his son to learn and grow. Only then will he truly find Nemo."

Personality
Marlin is a nice and caring parent to Nemo, but can also be a bit impatient and easily irritable.

Finding Nemo
In Finding Nemo, when Marlin's wife Coral and other unborn children were all killed by a barracuda, Marlin finds only one of his child egg. He names it Nemo, since that was one of the names his wife wanted to call one of her children. Through the years, Marlin has became overly protective over Nemo, much to the annoyance of Nemo. Marlin feels that the ocean is way too dangerous; he even doesn't want Nemo to go to school.

When Nemo gets captured by a pair of scuba divers (and after Nemo disobeys his father and swims up to a large boat known as a "butt" and touches it), Marlin risks everything to find his son and get him back. Along the way, he meets Dory who accompanies him on his journey.

After befriending some vegetarian sharks named Bruce (whom they first encountered), Anchor, and Chum, Marlin and Dory find a mask that Dory reads that says, "P. Sherman 42 Wallaby Way Sydney," but after reading the mask in another scene, a hungry anglerfish shows up, roars at Marlin and Dory, then chases after them. However, the duo manage to trap it by using the mask to tie it around a rock. Dory remembers what the mask says and on the way continues to repeat it until Marlin comes upon a group of Moonfishes and tries to ask them, but they only make fun of him, so Dory asks them for Marlin and they give them the directions to Sydney, to which Marlin thanks them. But unknown to the clownfish, the Moonfish also give Dory a warning not to swim over a trench. However, when Marlin and Dory reach the trench, Marlin makes Dory swim over a trench, much to her warning and they get stung by a group of jellyfish and knocked out. When Marlin wakes up, he finds that he's riding a sea turtle named Crush on the East Australian Current. Marlin asks Crush if Dory is okay and Crush points down to another turtle where Dory appears to be "dead". Marlin swims down to her, sees the scars on her side from the jellyfish stings, and blames himself for her getting hurt. But Dory wakes up from her "death" because she was playing Hide-and-Seek with the turtle kids. Marlin then tells Dory and the turtle kids about he and Dory's journey.

Later, after they left the East Australian Current, Marlin and Dory get swallowed by a whale, which was chasing krill.

Later on, when Marlin got sick of being stuck in the whale, he tried to break himself free, but fails, and he drops onto the whale's tongue, crying. Dory asks him if he's okay, but Marlin doesn't answer, so Dory swims down to him to comfort him and immediately says the same eight words "There, there. It's all right. It'll be okay" to do so. Marlin tells Dory his promise to Nemo, but Dory explains to her friend that he can't never let anything happen to him.

Trivia

 * 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 a actually a type of billfish (such as the sailfish and the swordfish) that is popularized by sport fishing.