Ace Killer (エースキラー Ēsu Kirā), is a robotic assassin and a major antagonist in Ultraman Ace. He served as the main antagonist of the two part episode "Execution! The Five Ultra Brothers" and "The Five Stars That Scattered Throughout the Galaxy", the main antagonist of the forty-third episode of Ultraman Mebius titled "Threatening Mebius-Killer", a major antagonist in Ultraman Mebius Gaiden: Ghost Rebirth, a supporting antagonist in Ultraman Zero Gaiden: Killer the Beatstar, a minor character in Ultraman Ginga, and a supporting antagonist in Ultra Fight Victory. It is a Choju created by Yapool specifically to counter and kill Ace.
Ace Killer returns in Ultraman Mebius, where he is used by Yapool to battle Mebius.
Ace Killer was created by Yapool to kill Ultraman Ace. After Ace Killer obtained the powers and weapons of the Ultra Brothersr, Yapool released a robot replica of Ultraman Ace to demonstrate Ace Killer's power by having Ace Killer destroy it.
Ultraman Ace attempted to rescue his brothers, but he was ambushed by the powered-up Ace Killer. Ace Killer easily overpowered Ultraman Ace with the Ultra Brothers' powers, but the Ultra Brothers used the last of their energy to empower Ultraman Ace. With the combined energy of his brothers, Ultraman Ace used a Space Q attack to destroy Ace Killer, freeing his fellow Ultras.
Ace Killer later returned when it was used by Yapool to combat Ultraman Mebius.
Yapool summoned Ace Killer to Earth to battle Mebius. Ace Killer easily had the upper hand in the fight, and things only got worse for Mebius when Yapool had Gadiba merge into Ace Killer to transform him into Mebius Killer. Mebius Killer brutally beat down Mebius. However, some encouragement from Aya gave Mebius the courage he needed to overpower and defeat Mebius Killer, destroying the robot with the Mebium Dynamite move.
Ultraman Mebius Gaiden: Ghost Reverse
After Yapool was revived, he used Ace Killer as a host.
Ultra Fight Victory
Yapool later used Ace Killer again, giving him a copy of Ultraman Victory's UITrans and declaring him Victory Killer. Ace Killer was sent to prevent Ultraman Ace from saving Ginga. Victory Killer was later destroyed by Victory himself and Shepardon.