Talassia has very long blond hair and brown eyes, made up of sky blue. Some strands of her hair are adorned with orange pearls. Its bra is very long and looks more like a tunic, sky blue, marked by cutouts on the side and a shell in spiral gilded as a brooch. She has pink ruffles on her arms and a long sky-blue mermaid tail adorned with pink ruffles.
Talassia is a very gentle and sweet mermaid. Like Coralia she is very fearful and shy.
Talassia is captured by Valtor, along with Coralia and Amarea. Valtor places his mark on them; stealing their magic and changing them into sea monsters that attacked anything that crossed their paths. After Valtor's defeat, Talassia's magic returned and she became a mermaid again.
Talassia appears again in "The Enchanted Island". The guardian of Tir Nan Og took the form of three mermaids from Andros; Talassia, Coralia and Nerimer. The Fake Talassia lures Aisha out into the water. After Aisha introduces herself, Coralia pushes Aisha underwater and with her fight against Winx in dungeon.
- In the Nickelodeon dub, she was called Nesia (not to be confused with the real Nesia).