Omnia

Omnia is the Supreme Guardian of Sirenix.

Season 5
In "Sirenix," the Winx restored the source of Lake Roccaluce after Tritannus stole it's power. The Sirenix guardians open the gate to the Infinite Ocean the, Omnia invited the Winx to enter the Sirenix gate.

Omnia tells the Winx that the Infinite Ocean is always open to them.

The Mystery of the Abyss
Omnia might make an appearance in the third Winx Club movie since she was seen in the trailer.

Appearance
Omnia looks like a larger, human-sized version of the Sirenix Guardians. She is yellow skinned and mermaid-like with a long and adorned tail. She has yellow hair shaped into an elaborate hairstyle, a pink shell-shaped tiara, some orange bracelets, yellow fins that form a cape behind her and a pink shell on her chest.

Trivia

 * She is a much larger version of the Sirenix Guardians.
 * Because of Omnia's yellow colour her appearance is similar to Stella's Sirenix Guardian.
 * "Omnia " is a Latin word meaning "Everything".
 * She and the Sirenix guardians look just like the undines that had appeared in the first season.