Leo is a Master Knight and was responsible for killing Blue Ring in the battle of Void. Despite being his first encounter with a EX-Type Zero, he won and earned the title of Zero Breaker. He is recognized as a strong knight by both Admiral Norton and Dry.

When he was a boy, Anne saved him and his sister in a warzone while she was still a trainee. Leo then became a knight and Anne's disciple, but this last is apparently a secret that he keeps from his sister. He holds Anne in high esteem and idolizes her as a knight, it is also shown that he has a badly disguised crush on her, but she keeps her distance and wants him only as a friend. He has fought side-by-side with Anne in the past. In the battle of Void he lends her his AB Sword so that she could finish off the Queen, after he breaks his right arm in his battle with Blue Ring.

At Anne's retirement party he had an minor scuffle with fellow Master Knight Daniel, but both were stopped by the Doll A-10. He plays a minor role in the Preemptive Strike on Baltia where it is revealed that he has been appointed a Bishop of the Crossleague.

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