Sir Aaron is a "legend" of the Kanto region. He was born in Cameran Palace and was a noble and honored knight. When two armies were about to fight, Sir Aaron went to the Tree of Beginning and sacrificed himself to save the kingdom. He has the ability to use aura, along with his friend Lucario, whom he traveled with.
Sir Aaron sealed Lucario inside his staff so Lucario wouldn't sacrifice himself to save the Cameran Palace, and that he could live in a time of peace. Ironically, Lucario ends up meeting the same fate when he gives his life energy to save the Mew and the Tree of Beginning one thousand years later.