Tritannus

Tritannus is the main enemy(if you dont count the trix) that the Winx face in Season 5. He was once a normal triton, the son of King Neptune and Queen Ligea, and the twin brother of Prince Nereus who was jealous that his brother had been chosen as future King of Andros instead of him. He tried to disrupt his brother's crowning ceremony, but was arrested and sent to prison. He later escaped to Earth and was turned into a monster by pollution and allied himself with the Trix. His goal is now to rule all the world and to conquer the Infinite Ocean and caused pollution so that he may grow stronger. The Winx go underwater to fight him, but their Believix powers are not enough. They will have to look for the Sirenix power and become mermaid fairies.

Season 5
He is introduced in Season 5 as the cousin of Aisha and as the son of King Neptune and Queen Ligea as well as the brother of Tressa and the twin brother of Nereus. His father was going to choose Nereus to become the crowned prince and the future king and Tritannus, who was absent from the crowning ceremony, "having made his choice" according to his father. He tried to assassinate his brother during the ceremony but was defeated by his father who had sent him to the prison of Andros, where he longer be known as king Neptune's son officially removing Tritannus' title as prince.

Later, an explosion occurred near Gardenia that caused an oil spill in the sea which reached the cell of Tritannus, turning him into a gigantic mutant monster and he, together with the Trix, escaped from prison and he stole the powers of the Selkies of Andros and of Earth, vowing to hunt down all the gatekeepers and to steal the powers of each of them so as to be able to enter the Infinite Ocean and to set to conquer the whole Magic Dimension from there. He then restored the powers of the Trix, which had been taken from them, but when he used up all the pollution he absorbed he lost his monster form and the powers he obtained from them.

He then went to Gardenia to absorb more toxic so as to become stronger and when the Trix sensed fairy magic there and found out that the Winx had brought magic back to Earth. Tritannus ordered his minions to destroy the Winx. The Winx defeated the minions and followed them, resulting in Bloom, Stella and Musa facing the Trix in an aerial battle above the sea and Aisha, Flora and Tecna facing Tritannus and his minions in an underwater battle, with Aisha fighting her cousin face-to-face. While Aisha recognized Tritannus as he began to lose his monster form after using up all the toxic he absorbed, Tritannus did not recognize her.

After escaping back to Andros (or possibly to seas of other realms of Magix) he was angry for his minions about chasing fish. After Icy told him they didn't have enough power to defeat the Winx' Believix powers, Tritannus gave Trix much more powers than before. At this point Tritannus was also obsessed of finding more toxic pollution.

Tritannus later learned about Sirenix from Icy. He went along with his minions to his former home to learn more about Sirenix when he ran into his twin brother Nereus and mother Ligea. Nereus tries in vain to convince his brother that he was once a good person, but transformed him in to a mutant Triton. He then forced his mom to tell him about Sirenix after threating to kill his brother. When his mother couldn't tell Tritannus the information he needed other than Daphne, he transformed her into a mutant mermaid just like the rest. He later contact Icy telling her about his plan to hunt Daphne. When Icy tells Tritannus where they can find Daphne, he smiled and laughed.

He later tells to the Trix to attack the Winx Club in Graynor. He is then seen in Andros, talking with his mother and brother on how he will reunite them with King Neptune and Tressa

He then goes to Solaria and absorbs Aliris' power.

In the eighth episode of Season 5, he travels to Melody and tries to turn Aisha into a monster, but turns Musa into one instead.

Ability
Tritannus in his triton form is incapable of using magic independently. Like King Neptune and Princess Tressa, he is quite a master in physical combat. His weapon is a trident. With his trident, he is able to cast average magic blasts and bursts.

He has ability to drain toxic pollution in order to gain magical powers and monster form, though its possible all magical sea-creatures possess that skill, but are trying to avoid touching pollution.

He is much more powerful in his monster form, gaining the ability to cast basic spells alone (like telekinesis) and gaining a certain resistance to magic. His magic blasts with the trident is much more destructive and carries the power to mutate other people it hits. However, he can only use so much of his monster power before being change back to a triton and have to look for more toxic to change again. But in later episodes, he appears to have not changed back to his original form but after he stole the Aliris' powers and entered the gate to Solaria he changed back and asked the Trix to find more toxins on earth.

He has every gatekeeper power from each of the Winx respective homeworld thus making him capable of entering the infinite ocean.

Personality Profile
He is shown to be really jealous toward his brother Nereus, who their father chose to be crowned prince and People of Andros, the mermaids and the mermen, like better than him. So he finally decided to attack him. In jail, he met the Trix and became good friends with Icy, both of them seeming to understand and to have romantic interest with each other not to mention Icy seems so far to be the only one w                ho Tritannus cares for.tritanius also has a crush on icy and will do anything to help her. icy will do the same. Aisha described him as a "psycho", which has been proven to be true.

Also he seems to be somewhat naive toward Icy and easily angered. He somewhat enjoys draining toxic to become an monster and when he decided to become even more powerful, he became a little obsessed of finding more oil or toxic chemicals. He was even willing to turn his twin brother and mother into mutant mermaid and Triton.

Triton
He looks identical to his twin brother, having long hair fair skin, a muscled body and long snake-like fish tail, but he has yellow eyes, a green dorsal fin and scales and darker purple hair. He has somewhat similar facials details as Valtor.

Monster
In his monster form, he is larger than in his triton form. His upper body has a reptilian appearance, with red eyes, purple lips and green skin.

Trivia

 * Tritannus' name appears to come from a mix of Triton and Titan.
 * Triton is a Marine God of Greek Mythology who is the son of Poseidon, the God of the Sea (whose Roman equivalent is named Neptune), and his wife Amphitrite, the Nereid sea nymph and daughter of the Sea God Nereus. Triton is depicted as being half-man half-fish, the tritons are his descendants and, like him, are also half-human half-fish. One of Amphitrite's 49 sisters is named Galatea.
 * Titans are a race of primeval deities in Greek Mythology who are said to be immortal giants and who once used to rule the world until the God Zeus defeated them and proclaimed himself and his brothers and sisters as the new rulers of the world.
 * Tritannus is the first villain ever to be family-related of one of the Winx, namely Aisha.
 * Tritannus is also the first main villain who does not have something to do with the past of a main character, assuming that Cassandra and Chimera from Season 3 are minor villains.
 * Tritannus may have been partly inspired from King Triton, the father of the mermaid Princess Ariel in the 1989 Disney film "The Little Mermaid" as they both have a trident as weapon.
 * His ability to drain toxic waste is similiar how Valtor drained magic power from magic treasures and Ogron drained fairy magic.
 * Tritannus and the Trix are the youngest main villians in the series.
 * He may have a crush on Icy.
 * He is disliked by Darcy and Stormy.