As a puppy, Ace lived in a pound, until he was sold by a corrupt attendant to Ronny Boxer, a man who trained and forced dogs to fight one another for the amusement of paying crowds. Boxer recognized a certain potential in the pup, and planned to train him to grow up "mean"—a fighter. Ace was defiant towards his new master, but ended up leading an abused, violent life in Boxer's dog-fighting ring.
One winter night, when Ace was fully grown, the police busted Boxer's game just before his first fight would have begun. Ace was freed from his former master, but also fled from the police when they tried to catch him. He wandered the streets of Gotham for a while, and soon encountered Bruce, a fellow tormented soul, who was making his yearly pilgrimage to the spot where his parents died. When a member of the Jokerz gang tried to mug Bruce, Ace helped fight him off, but was injured in the process. Bruce saved him and brought the dog to his home.
Ace and Bruce soon developed a close bond as Ace is shown to be Bruce's constant companion, as well as something of a guard dog. Bruce is shown to be quite protective over Ace, as when he became lost among the streets of Gotham and vowed to never let anyone else hurt Ace again, after discovering his past abuse. Aside from Bruce, young Terry McGinnis, Bruce's assistant and protégé, seemed to be the only other person with whom Ace was ever able to connect with. At first, Ace was untrusting towards Terry, but as time went on both Ace and Terry developed a close bond of their own.
Although Ace typically did not accompany his guardians into the field, the dog was often an asset when he did. For instance, his species' acute sense of hearing and smell and fighting experience were considerable assets. Furthermore, he was extremely strong for his species, capable of throwing a man a considerable distance with his mouth even with poor leverage, able to bite through an electrified manacle with ease, and even managed to stand up (albeit barely) after being electrocuted by Joker's buzzer, which has been known to knock out grown men with a simple touch.
Bruce could summon Ace to attack instantly by simply snapping his fingers. Ace was able to stay quiet and not draw attention to himself until the moment he was ordered to strike.
Ace is a homage to Ace the Bat-hound from DC Comics.
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