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The Blue Knight suddenly appears at the end of the fourth volume of the manga as an unknown alien male to rescue Ichigo during a battle with Kish. He tells Ichigo that he "is hers" and that he was "born to protect her". After his first appearance, the Blue Knight reappears whenever Ichigo is in danger to protect her, and disappears after the situation is resolved. As the series progresses, Ichigo attempts to learn his true identity and suspects both Ryo and Keiichiro, before learning near the end of the series that the Blue Knight is an alternate persona of her boyfriend, Masaya.
It is revealed that this persona was created out of his desire to protect Ichigo. After his true identity is revealed, the Blue Knight, now knowing he is Masaya, fights with Mew Ichigo and the Mew Mews, and it revealed that Masaya and the Blue Knight are both alternate personalities of their worst enemy: Deep Blue.