A.C. Eos

Arthur C. Eos (A. C. Eos), known as Akatsuki Shidō (暁シドウ) in Japan and often referred to by the nickname Ace, is a Satella Police Commandos officer. As one of Geo Stelar and Omega-Xis's allies, he can be fought. He can EM Wave Change with his Wizard, Acid, to become Acid Ace. The two are registered under the Satella Police's Project TC as No. 001. He is the creator of Hunter VG, the device that succeeds Transers and Star Carriers as well as allowing Wizards materialize.

Due to Acid being a man-made Wizard and not a natural EM Being, wave changing puts a tremendous strain on Ace's body to the point where Acid will voluntarily revert the wave change in order to protect Ace.

Mega Man Star Force 3
Geo Stelar first meets Ace at Echo Ridge Elementary where he asks Geo to enter a tournament being held at Spica Mall. Geo is late to the tournament that Ace won in a couple of seconds.

During the events of the game, A.C. Eos teams up with Mega Man and his friends to attempt to thwart Dealer's plans for world conquest. He provides Geo with the Ace Program or Joker Program (depending on game version) to allow Mega Man to control the influences of Noise. He also leads a team of Satella Commandos with wave change capabilities, comprised of Geo, Bud Bison, and Sonia Strumm. Later in the game, it is hinted and then completely revealed that he used to be a member of Dealer before leaving and joining the Satella Police. In the past, it seems that he used to have a romantic relationship with Queen Tia.

During the Dealer attack on WAZA, he defends the base and then fights against Queen Virgo. Virgo gains the upper hand early, but Ace turns the battle against her. However, his Wave Change is canceled because of the strain on his body. He then requests that Geo defeat Virgo, which he does. Ace supposedly dies when Dread Joker blows up in the Dealer's hideout, but manages to tell Tia that he loves her before he disappears.

A message gets sent to Geo from Ace, much to his surprise, and the message was programmed to automatically be sent to his friends and colleagues, as well as the Satella Police members. Ace tells Geo to not give up hope, and gives him the last battle card up his sleeve, Dread Joker.

Toward the end, when Crimson Dragon knocks Geo down and starts giving up on the fight, he sees Ace before him, asking why he has given up, when he is supposed to be taking his place in the fight. He tells him to not give up, because he is the hero of justice, after all!

Fans are skeptical because when the credits roll, an image of Ace can be seen in a hospital, being taken care of by Dr. Goodall and Queen Tia, suggesting they found data frags of him and reassembled him and Acid like they did with Luna. However, in the Mega Man Star Force Official Complete Works artbook, Yamashita, leader of the scenario team, implies that the image might be a representation of someone's desperate wish, and that Ace may not have survived at all, and they left it for the player to decide.

Trivia

 * After trying a Mega Snack, A.C. Eos becomes very fond of them. He is seen throughout the game munching on one or several whenever he has time to do so, to the point that it worries Acid. They can even be found under his desk.