Natassia (ナターシャ, Natāsha) is Cygnus Hyoga's mother, who died when the ship she travelled in, while on the way to see Mitsumasa Kido, Hyoga's father, sinks into the icy Siberian sea.
When the ship was sinking, Hyoga was evacuated but Natassia stayed in the doomed ship, going under with it. As the ship sunk in the icy sea, her body remained uncorrupted.
Having the ability to visit her frozen tomb was the real reason who drove Hyoga to became a Saint. After a hard training, Hyoga was able to break the layer of ice and swim to the ship's wreck, where saw the body of his mother, laying flowers upon her frozen tomb.
Natassia is a recurrent character in Hyoga's memories, and this was used many times by his enemies, to make him remember the grief he felt when he lost his mother.