Ace was a young man with purple eyes and short, straight, ash blonde hair, whose bangs parted in the middle of his face and a part of his hair covered his right eye. Like most of the mafia, he often dressed formally. He dressed in a gray suit vest and a purple tailcoat, as well as matching-colored slacks. He also sported a black necktie that was accessorized with an oval ruby partway in the middle, white gloves, and gold vertical bar earrings.
Though he was an executive, Ace had no loyalty towards the Port Mafia and even tried to recruit Fyodor in his attempt to rebel against the organization, particularly to assassinate Ōgai Mori. He was also quite cocky and confident in his own prowess and aptitude.
Ace took the lives of his men lightly, where he believed that turning their lifespans into jewels with his ability at will gave them purposeful value. Moreover, though formal in demeanor, he apparently saw mafia members as inferior to him. He also had a rather hostile relationship with fellow mafia executive Kōyō Ozaki and perceived Mori to be quite short-sighted and stupid for a leader. Amidst this imposing attitude, Kōyō noted that he was one of the first executives who fled during the Guild war, contrary to the other mafioso, commanders, and executives who personally went on the field to protect Yokohama's traffic systems.
Despite his arrogance, Ace was intelligent, observant, and cunningly skilled in gambling. Having his intelligence underestimated greatly insulted him, making him prone to bouts of violence as a result. With his sharpness, he noticed the faint inconsistencies of his base since Fyodor's arrival, ranging from the door lock to the frozen clock. Nevertheless, his overconfidence prevailed and ultimately led to his demise.
Ace's ability, The Madness of the Jewel King (宝石王の乱心, Hōseki-ō no Ranshin?), is an ability that turns his subordinates' life (those equipped with a special collar he gave) into jewels that are worth the target's lifespan.
Ace started as an astute, clever gambler who dominated one of the Port Mafia's casinos. He bribed authorities a great deal of money, quickly rising and buying his way to the rank of executive within the mafia. However, he recognized other mafia members as nothing more than mere bodyguards, with whom he shared mutual distrust towards, including his fellow mafia executives. At some point, Ace secretly compiled a list of the abilities of Port Mafia agents – including Mori's top-secret ability – for his supposed rebellion, which he kept in his jewel vault.
Ace reports the capture of the mastermind behind the fall of Moby Dick, Fyodor Dostoevsky, to Mori and Kōyō. He describes the man as a "soulless, vampire-like man", but he keeps his thoughts to himself rather than further elaborate to Mori.
Mori suggests leaving Fyodor to Kōyō and her torture squad, but Ace insists to take care of him himself. This surprises Kōyō, who points out how such executive who mostly stayed out of the war against the Guild can handle the matter. Ace retorts that it is better than being the first of the Port Mafia to be captured during the war, greatly antagonizing Kōyō. Before their debate goes any further, Mori abides and leaves Fyodor in Ace's hands, much to the latter's pleasure.
In the depths of the mafia's headquarters, Ace greets Fyodor and introduces himself. Taking a knife, he cuts off his restraints, ridiculing his subordinates for treating a "guest" in such a way. He immediately orders drinks and clothing for Fyodor.
After Fyodor is dressed, Ace claims to be a lonely man. The mafia distrusts him, and he distrusts the mafia. The only thing he trusts are his games of chance, the jewels in his vault, and his private force of fifty men. He refers to the man as the "51st" person he can trust and invites Fyodor to join forces with him in killing Mori.
Fyodor refuses and instead threatens to kill Ace. The mafioso snaps, breaking a bottle on Fyodor's head. Clenching his fist, Ace demonstrates his ability. One of his subordinates suddenly falls to his knees in pain, and Ace turns him into an array of jewels. He claims that his ability is one that overflows with benevolence and explains that the collars his subordinates wear are equipped out of their own agreement, which can never be taken off thereafter. He orders his remaining subordinates to clean Fyodor up and claims that, if he agrees to put a collar on, he shall not kill him, after which Ace takes his leave.
Some time later, when Ace returns, Fyodor suggests they play a game. Fyodor is confident Ace cannot kill him because Ace wants information on his group's next move and finances, logic that Ace understands. The mafioso then decides the rules of the game:
- Take one playing card at a time from the same deck.
- Guess if the card will be lower or higher than the previous card.
- If the guess is correct, the guesser goes again. If incorrect, the other player draws.
- When all cards are drawn, the player with the most cards wins.
- If Ace wins, Fyodor leaks information. If Fyodor wins, Ace grants him his freedom.
Fyodor draws first, guessing the card below the topmost card and the next correctly, both of which Ace counts only as luck. However, Fyodor continuously guesses correctly and ends up with all the cards, Ace never having a turn whatsoever. Throwing a fit, Ace orders for Fyodor to have his limbs cut off.
This proves impossible: Fyodor initiating the game was actually to antagonize Ace and stall for some time to have the Rats infiltrate the underground base. Fyodor bargains that, if Ace hands over the jewel vault's key, he will survive. Ace, undeterred, reveals that his base is not on mafia territory nor is it truly underground, but they are rather onboard at sea. Apparently, its equipment is such that counters ability users; hence, it is impossible for the Rats to take over their ship without causing an uproar.
Ace also reveals he wiretapped Fyodor's earlier conversation with Karma and knows that Fyodor can lock his enemy's consciousness inside his own head, which explains his luck in the card game, the radio that does not work, the door that cannot be opened, and the clock that has not been moving since his return to the room.
He then brags about overcoming Fyodor's ability, calling himself a "king by birth" thanks to knowing all there is to know about Fyodor as told to him by the man who captured him in the first place. With great pride, he reveals he knows how to free himself of Fyodor's ability: killing himself in the man's mind and waking up alive in the real world to kill the Russian. He eventually takes the wire off a lamp and hangs himself with it. In truth, after leaving the room of his own volition, Fyodor reveals to Karma that he tampered with the room all along and that his ability is not locking a person's consciousness in his own. Ultimately, this means that Ace hanged himself and died for nothing.
- Ace's name is also written as A, albeit pronounced as "ace" (ēsu?). This is most likely a reference to the ace playing card, which is also commonly denoted with the letter "A". This can also be a nod to his former status as a gambler who specialized in card games.
- His ability, The Madness of the Jewel King, refers to Ace himself, though its name most likely comes from the play The Madness of George III, a play by Alan Bennett. Likely to avoid copyright complications, Ace's ambiguous name and ability name focus strongly on the character rather than the author.
- Bungo Stray Dogs Manga: Chapter 42.