They look like miniature mermaids with transparent flaps of skin under their arms which allow them to fly, but have no real wings. They are usually purplish or bluish in color, except Lusiz, who is pale green, and the Queen of the Undines, who has golden-dullish light lime green skin. They appear to be of varies colors, the most prominent being a very light shade of blue. Their whole body is like a mermaid's however, they are much smaller in size and their hands, aside from their thumbs are fused together and have transparent flaps of skin under their arms.
- The Undines do not have wings, but instead have transparent flaps of skin under their arms which allow them to fly.
- They seem to have nature-based powers and to be very in touch with nature for the following reasons:
- Lusiz explained that while she was asleep under the Red Willow she was in telepathic contact with the environment around her.
- The Undines gave a Cheerful Gladiolus to Flora which they told her would allow itself to be picked only by her while Flora had earlier mentioned to the Winx that the Cheerful Gladiolus cannot be picked because it has thorns. This means that the Undines were able to get in touch with the Cheerful Gladiolus to convince it to allow itself to be picked by Flora. The Undines probably need to have nature-based powers to be able to contact the Cheerful Gladiolus in this way.
- The Undines can shine in the dark like fireflies and leave a trail of sparks behind them.