Kagemaru is the main antagonist from the first season of Yu-Gi-Oh! GX and he is the leader of the Seven Stars Assassins.
He was voiced by Michael Alston Baley in his old form and by Marc Thompson in his young form.
Kagemaru was once the superintendent of Duel Academy. When he left Duel Academy, he became ill and old. In terrible health, he now hopes to regain his health and youth by sucking the energy out of duel monster cards.
He wants to use the Three Sacred Beasts for his plan. The only problem is that they were locked underneath Duel Academy and protected by seven spirit keys which he once gave to Sheppard.
He sends the Seven Shadow Riders to win the spirit keys. When they all failed, he went to Duel Academia himself. He placed his body in a large machine which would infuse him with the power of the duel monster cards from all over the world once he had summoned the Three Sacred Beasts. While dueling Jaden Yuki, his youth did indeed return but once he was beaten, he regained his old age.
Kagemaru then promises to leave his evil ways behind him. His Dark Item was a ring. He plays a "Three Sacred Beasts" Deck, which revolves around summoning the Three Sacred Beasts. He returns in Season 4 as an advisor to the heroes.