Andy is a central character known for his powerful and peculiar nature. As a Negator, he possesses the ability "Undead," which completely negates his own death, making him truly immortal. This power prevents him from dying by any means, automatically regenerating any damage he sustains and even halting the aging process, leaving his appearance permanently frozen around his mid-twenties. His body can recover from virtually any wound, including being blown apart, and his severed parts disintegrate after about thirty seconds.

The defining aspect of Andy's personality is his reckless and death-seeking behavior. Having lived for centuries without the possibility of dying, he has grown weary of his endless existence and is obsessed with finding a way to achieve permanent death. He constantly seeks out new and exciting methods to die, a pursuit he describes with his characteristic exclamation, "Sick!". Despite this macabre obsession, he is not malevolent. He has a bold and uninhibited nature, showing no discomfort with nudity or intimate situations, yet he explicitly states he would never force himself on a woman who is unwilling. His long life has also honed his combat instincts, making him a highly analytical and creative fighter.

Andy's primary motivation throughout the story is to find a "greatest death" that can finally overcome his immortality. He believes that Fuuko Izumo's ability, "Unluck," which brings catastrophic misfortune to anyone who touches her skin, might be the key to achieving this goal. This belief drives him to initially partner with Fuuko, though his intentions evolve beyond selfishness as their journey progresses. His role in the narrative is that of a protector and partner to Fuuko, as well as a formidable frontline combatant for the Union, an organization of Negators. He holds the ninth seat at the Union's Round Table.

Andy's most significant relationship is with Fuuko Izumo. While he first sees her as a tool for his own death, he quickly becomes her dedicated guardian, willing to risk himself to keep her safe. A complex dynamic also exists within his own mind, as he harbors a second personality named Victor. Victor is Andy's original personality, a far more ruthless and battle-hardened being who has lived for billions of years. Victor is sealed away by a metal card embedded in Andy's forehead; removing the card allows this alternate, more violent personality to surface. Andy also shares a notable past with fellow Union member Gina Chamber, who fell in love with him during his captivity fifty years prior to the main story.

Over the course of the story, Andy undergoes significant development. He begins as a suicidal loner focused only on his own end but gradually forms genuine bonds, most notably with Fuuko. His care for her shifts his perspective, giving him something to live for even as he seeks to die. His relationship with his other self, Victor, also evolves from one of suppression to a more nuanced coexistence. A running gag in the series involves his clothes being destroyed in battle, which he cannot regenerate. This problem is eventually solved when he tames the UMA Clothes, which provides him with a self-repairing suit.

Andy's combat abilities are almost entirely derived from his creative use of his regeneration. He has developed a unique fighting style known as "Bloody Arts," which incorporates his body's rapid healing into powerful and unorthodox attacks. His signature techniques include "Parts Bullet," where he severs and then launches his own body parts as projectiles at high speed, and "Repair Boost," which uses the explosive force of his regeneration to amplify the speed and power of his sword strikes, such as in his "Crimson Crescent Moon" iaido technique. He wields a katana that is indestructible, a weapon he stole from a Union member who had the power "Unbreakable".