He is an elderly wise loggerhead sea turtle who trains Pi in standng up to Troy.
He is voiced by Rob Schneider (who also voiced Bart & Eddie and many other characters in the movie).
Nerissa is a grumpy and mysterious loner. The reef fish gossip that he's some kind of wizard, and the only fish who are still real close to him are the marlin trio Moe, Manny & Jack.
One day Pi, and Dylan witness the turtle facing squid thieves trying to steal his blue pearl. Nerissa easily out wits the squids and recovers his pearl. When he sees the two witnesses, they both quickly leave.
Nerissa later tends to Pi when Troy beats him up and forces Cordelia to accept the shark's pearl if he leaves Pi alone. Pi requests Nerisssa to help him fight Troy, but Nerissa refuses and Pi leaves.
Later, after witnessing Pi scare off Bart & Eddie who were going to steal the turtle's pearl, Nerissa decides to train Pi.
After a few days Pi gets frustrated and accidentally knocks Nerissa's blue pearl off the table. Nerissa reveals to Pi that he's like this because his wife was killed by fishermen and all Nerissa has left is the blue pearl he proposed to her with. Nerissa admits to Pi that he never agreed to teach him how to fight Troy, but how to defeat him.
At the climax, Troy bashes Pi to a wall, causing rocks to fall on the fish, Nerissa rives to help but is whacked by Troy who then Threatens the turtles, life but the marlins distract the shark. When Nerissa asked what they were doin Moe replies "helping out an old fiend like we should've done a long time ago."
After Pi defeats Troy, Nerissa thanks the marlins for helping him. Nerissa then gives Pi his blue pearl for him to propsee to Cordelia to which Pi does.
The Reef 2: High Tide
In the Sequel, Nerissa tells Pi that he can't defeat Troy on his own this time. The other fish must also defend the reef. Troy has Ronny, a dwarf shark disguise himself and sabatoge Pi's training with a talent show. When Pi confronts Nerissa about this, Nerissa suggests that Ronny perhaps "offers something you haven't like a chance to prove themselves." Pi retorts that he tried train them to which Nerissa responds "I suggested that you teach them to be the best they can be--not to be you." He then shows Pi an image of the legendary Sea Dragon, which those who control it must be worthy to command it. When Pi asks him if he's ever tried summoning the Sea Dragon, Nerissa responds "many times."
Nerissa later goes to a mysterious cave and prays to become "prove to be worthy of your help."
Later, Troy and his goons ambush Pi and the evil shark threatens the fish that he's gonna destroy the reef tomorrow and that "shame you're gonna miss it." Troy bares his teeth, but Nerissa arrives from behind Troy and uses a sandstorm to make Pi literally dissapear from the sharks. As Nerissa gives Pi some cure for his headache, they both see Bart & Eddie capturing Ronny when he's exposed as a shark and bring him to Pi. But Ronny remorsefully tells that Troy was forcing him and that he'll help Pi get Cordelia back from Troy. Pi, Nerissa, Bart and Eddie decide to give the remorseful dwarf shark a second chance.
As the other fish try to prepare for the shark battle, Nerissa enters a mysterious volcano to summon the sea dragon, and mysterious powers flow around the turtle as he summons the mysterious dragon.
At the climax, Troy subdues Pi but before he can kill him, Nerissa shows up with his powerful sea dragon and blasts the other sharks. But Troy sneaks up from behind the turtle and knocks him off the dragon, causing it to dissolve away. Troy then prepares to kill Nerissa but Pi throws a Water Ball at him, and Troy returns his attention to his sole nemesis and gives chase to him.
After Troy is sent flying out of the sea, Nerissa congradulates Pi for becoming a true leader. Pi and the gang thank Nerissa for risking his life for all of them to which the turtle responds "It was my part to play." Nerissa then dances with the other fish, just as Ronny joins the party.