Don Avido is the Altar Black Saint, as well as the leader of the Nero organization, and former apprentice of Altar Hakurei .
Initially, Avido was an apprentice of Hakurei, who taught him the Praesepe combat technique, which he mastered. However, Hakurei learned that Avido surrendered to greed, and drove him out of the Sanctuary.
Avido then became a Black Saint and formed a criminal organization called Nero, a XVII Century mafia centered in Venice, with the objective to rule the world and destroy the Sanctuary. He still bears a grudge against his former master and believes the Sanctuary to be unworthy of ruling the world, as they are devoid of desires. He has mastered the Praesepe combat technique, commonly associated with the Cancer Gold Saint, as proven by his usage of the Praesepe Demonic Blue Flames on Lumaca’s spirit, he also is able to use the technique to destroy not only the body but also the soul of his victim.