Mai Valentine, known as Mai Kujaku in the original Japanese version, is a character who begins her journey as a skilled and confident Duelist motivated by prize money and personal pride. Her past reveals a solitary nature, having spent much of her life alone and working as a casino dealer on a cruise ship. In that environment, she encountered many wealthy men and grew to distrust others, developing a manipulative edge to beat them at cards using a technique called Aroma Tactics. This strategy involved scenting her cards with different perfumes, allowing her to identify them without looking, which gave her a deceptive advantage.

This independent and wary personality defines her early appearances. She is self-absorbed, viewing the Duelist Kingdom tournament primarily as a means to win the substantial cash prize to fund a luxurious lifestyle. She believes she does not need friends, once commenting that her Harpie Lady cards were the closest she would ever have to companions. Her motivations, however, run deeper than mere greed. She also entered the tournament hoping to rediscover something she once cherished, a hint that her tough exterior masks a search for meaning. Her role in the story is that of a rival and occasional ally. She first challenges Joey Wheeler, mocking his inexperience, but loses to him in a pivotal match where he deduces her Aroma Tactics. This defeat, and her subsequent interactions with Yugi Mutou and his friends, slowly begin to chip away at her hardened shell.

Key relationships profoundly shape her development. Her connection with Joey Wheeler is central; she develops a soft spot for him, and he becomes someone whose respect she craves. Yugi Mutou also earns her admiration for his pure dueling spirit. Through them, she starts to care about the deeper meanings of the game and the value of camaraderie, moving beyond selfish goals. However, her progress is not a straight line. During the Battle City tournament, her feelings of loneliness resurface, especially when she believes her friends do not value her. The villain Yami Marik exploits this vulnerability during their duel, using psychological torment to break her spirit. Although she fights back and even manages to summon the Egyptian God Card, The Winged Dragon of Ra, she cannot control it and loses, with her mind being trapped in the Shadow Realm. This traumatic event causes her to relapse into her old, distrustful ways.

Her development takes a darker turn in the Waking the Dragons arc. Reeling from her defeat and haunted by nightmares, she is found by a man named Valon and manipulated by the villain Dartz. Convinced that her friends have abandoned her, she joins the Doma organization and is corrupted by the power of the Seal of Orichalcos. Her personality becomes ruthless as she duels against her former friends, seeking revenge for her perceived pain. It is only through a duel with Joey that she finally breaks free of the seal's control, remembering their true bonds. In a final act of redemption, she attempts to confront Dartz herself but is defeated, losing her soul in the process. Though she is later revived, she spends the remainder of the series alone, re-evaluating her life and her passion for dueling, promising to return one day.

Notable abilities extend beyond her dueling strategies. While her Aroma Tactics is her signature technique in the first tournament, she is a naturally gifted player with a tactical mind. Her decks are consistently built around the Harpie Lady archetype, focusing on swarming the field with Harpie Ladies and their evolved forms, Harpie Lady Sisters, supported by spells and traps like Elegant Egotist, Cyber Shield, and Mirror Wall. In later arcs, she incorporates Amazoness monsters into her strategy and, during her time under the Orichalcos's influence, uses powerful cards like Harpie Lady Phoenix Formation to control the battlefield. She stands 175cm tall and has an O blood type. Her English name is a pun on "my valentine."