Thalassa is a woman of indeterminate origin who appears to be in her late twenties. She stands about five and a half feet tall, and looks like someone who spends most of her life outdoors. Her skin is light green, with darker patterns visible on the hands and neck. A few brighter patterns are scattered across her nose and cheeks. Her hair is dark green and plaited into braids that fall past her shoulders. Her eyes are very dark, adding to her usually serious expression.

In civilized areas, Thalassa wears a mismatched set of simple but sturdy clothes that appear to have come from various places around the Mediterranean Sea. They look somewhat frayed and faded, but any damage has been carefully mended. Around her neck, Thalassa wears some kind of charm or amulet made from leathery green leaves.