Saizo

Saizo and his younger twin brother Kaze are members of a ninja clan from Igasato that has proudly served the Hoshidan royal family for several generations. Their father, Saizo the Fourth, was killed by Kotaro in an unexplained incident. In truth, Saizo is not his birth name, but rather, the title he was given when he became the leader of his clan as a result of his father's death. At an unstated point of time, he traveled to Mokushu in search of his father's killer. Although he managed to locate the culprit, he failed to assassinate him and was instead captured by the Mokushujin. Presumably tortured during his captivity, Saizo barely managed to escape with a scar imprinted on his face and his right eye lost.
Similar to Kaze, Saizo is endowed with good looks that are considered attractive by women. Disliking the attention that his facial features can potentially attract, Saizo thus conceals his face with a mask.

(Source: Fire Emblem Wiki)