Lusiz is an Undine of the Black Mud Swamp.
When the Winx accidentally meet the Undines in the Black Mud Swamp, their Queen tells them that they need a plant called the Xylith to make their homes but that it grows on the Black Island in the swamp and that there is a dangerous monster near the island. She also tells them that sometimes one of the Undines volunteers to collect some Xylith leaves and that the last one who volunteered, Lusiz, never came back.
The Winx the go to the island to defeat the monster but find Red Willows that lead them to their leader who emits a reddish sleeping gas that makes all of the fall asleep except Tecna who, with Bloom's help, uses her powers to create a shield where the gas cannot affect them and awake the other Winx and all the sleeping creatures on island. They find Lusiz at the foot of the leader Red Willow and when she wakes up she tells them that she was in telepathic contact with the nature around her and found out that the monster was in fact a giant turtle who was asleep because of the Red Willow and who would sleepwalk, so that its head moving in its sleep appeared to be a giant monster.
The leader Red Willow then attacks the Winx but Bloom and Flora destroy it and free the turtle and everyone on the island from the sleeping gas. Lusiz returns to the village of the Undines where the Queen tells them that before leaving the turtle allowed them to collect as many Xylith leaves as they could but that they would still eventually run out of leaves and would end up by being vulnerable to predators. Flora then uses a rapid-growth formula she made to make countless Xylith leaves grow so that the Undines will never run out of it and will always be safe. The Undines then give Flora the Cheerful Gladiolus, a magical flower she was looking for, as a present for helping them.
Lusiz, like all the Undines, looks like a miniature mermaid with flaps of transparent skin under her arms that allow her to fly and all her fingers except the thumb are fused together. However, unlike most other Undines, her is greenish.
Lusiz is kind and serene and appears to be very positive. She cares about nature around her and is cheerful.
Lusiz can fly thanks to the flaps of skin under her arms and she most likely, like all the other Undines, can shine like a firefly in the dark and leave a trail of sparks behind her.
She might have nature-based powers since she told the Winx that when she was asleep on the turtle she was in telepathic contact with nature around her.
- Lusiz might be derived from the name Lucy, which is derived from the Latin name Lucius and means Light.