He was voiced by Alexander Gould in the first film and Hayden Rolence in sequel.
When Nemo was still an egg, his mother Coral and the other eggs were eaten by a barracuda. Nemo survived this with a slightly damaged shell and was found by his father Marlin.
First Day of School and Kidnapped
Six years later, Nemo is now a mischievous clownfish with a small fin due to the barracuda incident and wants to have school. After waking Marlin, the two fish go to school and meet the young sea creatures. He goes to school with Mr. Ray and later meets three classmates (Tad, Sheldon, and Pearl) who go to open water after being bored. Marlin warned him that he is too young to go to open water, and Nemo retaliate by pointing his father's cowardly side, unaware that Marlin was overprotective on him out of trauma due to Coral's death.
After feeling embarrassed, the angered Nemo swims out into open water to touch a nearby boat, against Marlin's warning which only provoke him instead. With everyone were closer with open sea for too long, this attract the attention of 2 divers whom approaching them. In the chaos, Nemo gets captured by one of the divers, whom later revealed to be a dentist named Phillip Sherman and is kept on his aquarium where he met the Tank Gang and the pelican Nigel.
The Tank Gang help him try to do the escape plan where as stated by Gill, his small size allowed him to slithered through tube in aquarium as means to sabotage the aquarium's filter. The first attempt was ended with failure, but after heard the news about adventures that Marlin has gone through, Nemo started out the second attempt which proved to be successful. Unfortunately, on the scheduled day for escape that has arranged by the Tank Gang, Sherman has installed a new device that would rendered their recent escape plan failed unless thet destroyed it too.
Reunion and Saving Dory
When Marlin and Dory get help from Nigel, the plan failed due to Marlin's desperation wind up ensuing chaos that forced Nigel to retreated. When Nemo gets carried in a bag (thinking he's dead), he is later grabbed by Phillip's niece Darla and then being shook. The Tank Gang's leader Gill stops her and then catapults Nemo into the drain. It leads him back to the ocean and later is lost with the sewer crabs.
When Marlin and Dory find Nemo, they are lost with the groupers being caught by the fishermen. Nemo later gets in the net to free Dory in order for all the groupers to swim faster to free themselves. After that, Nemo is seen injured from the fallen net. Marlin uses his flashback and brings Nemo back to their home. After that, Nemo has a life that he will never misbehave with Marlin and goes to school with Tad, Sheldon, Pearl and a new classmate Squirt who is the son of Crush a turtle Marlin and Dory befriend.
Nemo is set to reappear in the sequel with Hayden Rolence doing the part, since Alexander Gould's voice has deepened too much for him to reprise the role. He helps his father find Dory's parents in the film and obviously more determined than Merlin in doing so, though the effort to help her is turned out not easy as it seemed.