Ace was originally an outsider brought to Wonderland by Julius, who at the time held a minor role with few duties, allowing him to raise Ace without interference. After Julius died under unclear circumstances during Ace’s teenage years, Ace entered a berserk state and killed the executioner sent to retrieve Julius’s clock. This act drew the attention of a White Joker, who, impressed by Ace’s display, offered him the now-vacant executioner position. Ace accepted on one condition: that Julius be instated as the Clockmaker, a role permitting his existence to persist across parallel worlds, even though most versions of Julius were already dead or doomed. To prove his readiness for a role demanding he kill anyone at any time, the Joker tasked Ace with killing another version of Julius. Ace traveled to a different world, killed that Julius, and allowed a younger version of himself to witness the act. He then mocked his child self’s helplessness, an expression of self-hatred. Ace later journeyed to the world where the events of Heart, Clover, and Joker take place, offering assistance to that world’s Clockmaker Julius. Over time, he came to realize this Julius was a distinct individual shaped by different experiences, and his efforts might have been futile, since the last remaining Julius might not retain memories of other people. In the world of Diamond no Kuni no Alice ~Wonderful Mirror World~, a younger Ace lives with Julius, who is already the Clockmaker. This younger Ace has not yet gained a role, and his foresight about his future as an official is blocked by the original Ace’s actions. Aware of their fated roles, Julius attempts to delay the younger Ace’s discovery by distancing himself, hoping to weaken their attachment and avoid accelerating the process in the Graveyard/Museum environment.

Ace serves as the Knight of Hearts and is subordinate to Vivaldi. He is consistently depicted with a smile and possesses a notoriously poor sense of direction, frequently causing him to wander outside the castle. He also works as a warden in Joker’s Prison but dislikes the job, as it interferes with his duties for Julius. His appearance includes tousled light brown hair, red eyes, and attire typically consisting of a red trench coat, black shirt, black boots with gray accents, gloves, and gold buckles on his arms and legs, along with a gold sword hilt and buttons. For meetings, he swaps the black shirt for a white one and adds a red tie. While working for Julius, he wears a light beige cloak, often stained with blood, and a small mask covering his eyes.

His personality is bright and fresh on the surface but reveals a darker side, particularly when the land shifts to Clover and Julius is absent. He can transition seamlessly from kindness to attempting murder without losing his smile, a trait that classifies him as a yandere. He is insightful and enjoys twisting others’ words, especially Peter’s and Alice’s. Though he often insults Peter and Vivaldi, he maintains a polite and respectful tone. He enjoys fighting and provoking others, such as Peter and Gray, referring to these encounters as training. His smile vanishes only when Alice is in serious danger, and his demeanor becomes frightening. He avoids actions he deems unknightly, often manipulating language to justify his desires. He hates rabbits, except as food, and is skilled at outdoor cooking. He excels at memory games and refers to most people with the polite honorific -san, except for Alice and Julius, indicating a desire to maintain distance. He plays dumb to irritate Peter or Vivaldi or to evade Alice’s questions. He ranks among Wonderland’s best fighters, typically only defeatable by Gray, though he can still win if feeling stable due to Julius’s proximity. He rarely instigates fights in games but is the instigator in spars with Gray. Like Alice, he can hear doors calling to him when truly lost, and he avoids these doors to prevent being brought to Julius.

Across various media, his interactions and role differ. In Heart no Kuni no Alice and Anniversary no Kuni no Alice, he first meets Alice in the garden of the Castle of Hearts, and she helps him navigate the maze. In Clover no Kuni no Alice, he lives at the Castle of Hearts but misses those left behind after the move, sharing this sense of loss with Alice since Julius is gone. In Diamond no Kuni no Alice, a child version of Ace exists without a role, looked after by Julius, and has not returned from a trip. In the manga Ace of Hearts, he travels with Alice through Clover, grows jealous of her kindness to others, and they eventually become a couple after he overhears her confession. In Love Labyrinth of Thorns, he tries to unite Alice and Julius, saves Alice to protect Julius, and sets off traps in a castle. In The Clockmaker’s Story, he acts as a supportive friend to Julius and defends him from threats. In Heart no Kuni no Alice ~Wonderful Wonder World~, he guides Alice but gets lost, flirts with her while claiming not to care, and decides not to kill her after finding her interesting. In the movie, he kidnaps Alice to provoke Peter, accidentally drops her from a chandelier, and is later revealed to be working for Joker, approaching Alice covered in blood before she is saved by Blood Dupre.