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Nemo is a very energetic young clownfish. He's very eager to go to school at the beginning of the movie, but lazy at the end. He is also quite friendly with anyone he meets. His father's overprotectiveness leads to him being frustrated most of the time. He leads him into being captured by fishermen. Adventuring and exploring are some of his likes.
He can also be very brave, as shown when he was willing to risk his life to jam the filter after hearing about all his father had done to come and save him and when he willingly entered a net that captured a school of fish to lead them to freedom.
His relationship with his father is what defines the film. Although Nemo loves his father, initially, he felt suffocated and confined by his father's overprotectiveness, which led him to tell Marlin "I hate you." However, after being kidnapped by some scuba divers and placed in a fish tank, he felt sorry for what he did but did not believe his father would come to get him due to his paranoia and fear of danger. However, after learning of his father's bravery, he became braver and hopeful, wanting to quickly meet up with and reconcile with his father, which he managed to do at the end of the film.
He also demonstrated remarkable leadership, as shown when he led a school of fish to direct them to swim downward to break the net that captured them.
Nemo is a young clownfish who lives with his father, Marlin in a sea anemone. Before he hatched from his egg, Nemo's mother, Coral, and the other eggs were killed in an attack by a barracuda. Only Nemo's egg survived, albeit slightly cracked. As a result of the crack, one of Nemo's fins is smaller than the others, thus Marlin worries about him. One day, Nemo heads off to his first day of school. At one point, he boldly leaves the reef to go to a boat on the open ocean. Marlin, who is watching (and also alerted by the student Kathy), orders Nemo to come back, when Nemo is suddenly taken by a diver to a fish tank in a dentist's office in Sydney, Australia.
In the tank, Nemo meets the Tank Gang, and their leader, Gill. The fish are horrified when they learn that Nemo, whom they have dubbed "Sharkbait," is to be a gift for the dentist's niece, Darla. Darla has a past history as a "fish-killer" and so the Tank Gang decides to help Nemo avoid that fate. The initial plan to make the tank dirty fails when the dentist installs a new cleaner. While the plan is being carried out, however, Nemo learns from Nigel the pelican that his father is looking for him, which lifts the young fish's spirits.
When Darla arrives, Nemo is placed in a bag. Nemo is able to fool the dentist by playing dead, causing the dentist to flush Nemo into a drain. However, an observing Marlin believes that Nemo has really died and swims off. In the ocean, Nemo runs into Dory, who has earlier helped Marlin in his search for Nemo. Though Dory does not initially remember Marlin's goal, due to a short-term memory loss issue, she soon recalls it and leads Nemo to Marlin who is still brokenhearted.
Marlin and Nemo are thrilled to see each other, but Dory is suddenly caught in a fisherman's net with a school of grouper. Nemo is able to use the skills he obtained from his time in the tank to save Dory. This gives Marlin a new-found respect for his son's abilities, and Marlin becomes less protective of him. He smiles as he watches Nemo going to school.
Nemo reappears in the sequel where he is neighbors with Dory and actually tolerates her memory condition. After a field trip with his school, he hears an unconscious Dory talk about the Jewel of Morocco Bay, California and when this escalates to Dory remembering her parents, Nemo is willing to help in contrast to his more reluctant dad.
When they arrive, they are attacked by a Giant Squid awoken by Dory, resulting in Nemo getting injured, causing Marlin to snap and insult her greatly, causing her to swim off in despondency. Needless to say, Nemo heard his dad and becomes quite upset with him for this for a majority of the film.
Nemo, in turn, is actually Latin for "no one." In the aforementioned novel, Captain Nemo was so called since nobody ever knows what his real name is if he ever had one. For the record, he was an Indian prince named Dakkar.
This could also be why Nemo is not included on posters for the first movie.
Nemo's friend Sandy Plankton could be a reference to Nickelodeon's SpongeBob SquarePants franchise, as "Sandy" and "Plankton" are names of characters from that show.