Nereus

Crown Prince Nereus is a triton. He is the son of Neptune, the King of the Seas on Andros and of Ligea, Queen of mermaids, and is the brother of Tressa and is the twin brother of Tritannus, who is the main antagonist in Season 5. He is also the cousin of Aisha and the nephew of Teredor and Niobe. When his father chose to name him as heir to the throne, his jealous brother Tritannus disrupted the ceremony and was sent to prison.

Triton
He closely resembles his twin brother including markings on his chest and forehead, muscular build, long hair and fair skin. However, his hair color is a light lavender that is also the same color as his dorsal fin, he has light colored eyes and a long blue tail. He wears a blue colored crested gem and an arm bracelet on both his forearms.

Mutant
After Tritannus mutated him, he looks identical to Tritannus' other mutants in which his build becomes skinny, has an ill colored skin tone with many wrinkles to his face and transparent flaps beneath his arms.

Personality
Unlike his brother who is vengeful and cruel, Nereus is kind and caring. However, this side of him causes him to not be able to accept certain things. For instance, he felt that Tritannus just needed to be spoken to rather than a physical confrontation, only to be proven wrong when Tritannus mutated him. He also appears to believe in second chances, shown when his brother had been captured, he permitted him to speak, testifying his sentence but was unsuccessful.

Season 5
In "The Spill", King Neptune was having a ceremony to name Nereus as the next king. But a masked assassin disrupted the ceremony and attacked Nereus. His father later defeated the assassin and found out it was Tritannus. King Neptune sent Tritannus to the prison of Andros.

He, his mother, and sister tried in vain to prevent King Neptune from going after Tritannus.

When Nereus and his mother went looking for Tritannus, they were turned into mutant tritons like the prison guards and continues to follow his orders.

Later in "The Eclipse", Aisha de-mutates him and his sister, with the power of his father's sword to which she also gives to him.

In "The Devourer", he and his sister attends the council being held in Domino's palace, he voices their need to protect the Pillars of the Infinite Ocean. This debate continues onward to the next episode, where he continues to voice out his opinion and gets irritated by Erendor's unwillingness to cooperate. He also returns home after Tritannus steals the seal from the pillar of balance to protect his citizens.

In episode 25, he is fatally wounded by Tritannus during a fight. Aisha acts quick, using her only wish to save her cousin, and revive him. The episode ends with Bloom, Flora, the selkies, and himself swimming after Tritannus, who has captured Aisha and swam away with her.

In the season finale, he and Bloom battle Tritannus. And although he puts up a good fight he is overwhelmed by Tritannus' power that was strengthened by the Emperor's Throne and it was up to Bloom to defeat him. After finally stopping Tritannus' he surfaces along with him, Bloom and Daphne and gives his brother one last chance to redeem himself. But Tritannus only scoffs at his own failure to annihilate them and is then sentence to Oblivion. Nereus then attends the celebration with his family held near the palace of Andros.

Trivia

 * According to Greek mythology, Nereus is a marine God. He is the son of Pontus, the Sea, and of Gaia, the Earth. He was sometimes called the Old Man of the Sea and with his wife is, Doris, who represented the bounty of the sea and who is an Oceanid nymph, one of the 3000 daughters of the God Oceanus, the World Ocean, and of the Goddess Tethys, the Mother of All Rivers and the Embodiment of all Waters of the World, who are both the children of Gaia and of Uranus, the Sky. With her he had fifty daughters, the nymphs called Nereids, with whom he lived in the Aegean Sea. Among them, Thetis was the mother of the Greek hero Achilles. Another of them is Amphitrite, the wife of Poseidon (the Greek equivalent of the Roman God Neptune), the God of the Sea, and Poseidon and Amphitrite were the parents of the half-man half-fish marine God Triton.
 * Nereus' voice actor also plays Sky in the 3D movies in the Italian dub.