Karel

Born in a family where only one can inherit the sword, and killed each other over it, Karel cut down his father and mother, and was a sole survivor, with the exception of his sister, whom he saw as no threat at the time. Afterwards, as if possessed by the sword, he went and slew Kelles –the Rock Giant of the Western Isles (said to be three times the size of a regular man), as well as Ilia’s Ice Emperor (a wizard that could freeze blood in a person's veins) among other strong opponents. From these feats, he earned infamy across Elibe, as well as the fearsome reputation as the "Sword Demon".

He encountered Eliwood while wandering and seeking opponents to fight, and joined his group in order to observe his skill.

(Source: Fire Emblem Wiki)