Nightmare

From Soul Calibur - Wiki on Neoseeker

Nightmare
Soul Calibur IV costume
Age	Unknown
Birthdate	Unknown
Blood Type	AB
Family	None
Fighting Style	Fighting styles stored in the memory of Soul Edge and self taught moves
Gender	Male
Height	Unknown
Nationality	Unknown
Weapon	Soul Edge
Weapon Name	Soul Edge
Weight	Unknown

Nightmare is the main antagonist of the Soul Calibur series. Nightmare is the dark side of Siegfried. After Siegfried obtained Soul Edge, he was consumed by the dark power of the evil blade. This caused his armor to be possessed by the soul of Soul Edge, known as Nightmare. Nightmare then grabbed Soul Edge and left Siegfried to die. Somehow, Siegfried managed to survive, and had a revelation and decided to seek redemption for his previous wrong doings. Now Siegfried is always on the move to find Nightmare, to get his revenge, and to redeem himself.

Nightmare has many nicknames throughout Tales of Souls mode, but the main one is the "azure knight." Obviously, because his armor is blue.