Knight Leader is a British noble whose real name hasn't been revealed.

He is the captain and leader of the Knights of England, one of the three organizations in charge of Britain's magical affairs along with the Royal Family and Necessarius. He and the knights are allied with the forces of Britain's second princess Carissa during the English Magic Civil War and thus is pitted against her sister, the third princess Villian, whom he would normally be a friend of.

(Source: Toaru Majutsu no Index Wikia)