She only appears in-game after the player reaches Stage 10.
The beautiful and bold Mai Valentine has traveled the world, taking down all challengers along the way. Though she may seem bossy and tough, she also has a softer side, like an older sister that you can count on (But don't call her "old!"). When Dueling, her "Harpie Lady" dazzles her opponents into defeat.
After the player advances to Stage 10, they will encounter Mai for the first time, who is engaged in a Duel with Jay. After Mai's apparent victory, she comments that she originally had high expectations when she heard all the top Duelists would be in Duel World, and wonders if there is anyone in Duel World who can give her a challenge.
Until the player defeats Mai at least once, whenever they challenge Mai to a Duel, she remarks that the only way to get further is by defeating her. She then observes that the player is already wondering what the next Stage will be like, and chides them for thinking too far ahead; instead, she prompts the player to never take their eyes off their opponent, especially since it's her. She concludes her dialogue by stating she's definitely in the mood for a Duel.
Until the player defeats Mai at least once, whenever they lose to her, she will express disappointment and claim their Duel was so easy that she could have won it in her sleep. However since she's in a good mood, she tells the player, she'll give them some advice: a Duelist's spirit isn't tested when they win, but rather when they lose. In other words, she says that if a Duelist has heart, then a defeat won't break them, before rhetorically asking the player if they have heart. At this point, she says she's given enough advice to the player, and that she hopes the player will give her a challenge the next time they Duel her.
Upon Mai's first defeat, she states that she's impressed, but that it doesn't mean she'll give up her dream. She goes on to state that a Duelist's courage is put to the test after a defeat, not a victory, and that she is confident that her loss will strengthen her, leading her to achieve her dream. Mai then leaves the player with a warning that she'll be much tougher to defeat the next time they Duel.
After the player redeems their final unclaimed reward for Mai Valentine's Character Unlock Missions, they will encounter Mai again, who declares that because her friends are always around to help her overcome any obstacle, her spirit won't be broken that easily, and that she'll fight to the end. She tells the player to watch as her monsters dance their way to victory, and shows them her "Harpie Lady Sisters" card.
Most of the time when she declares an attack with one of those monsters, she announces "Let's go, Harpie Lady!" followed by "Scratch Slash!"
Whenever Mai Valentine Summons "Harpie Lady Sisters", a cut-in frame of Mai will briefly appear, and she'll announce "Witness the trio of terror!" followed by "Harpie Lady Sisters!" If that player hasn't already Summoned "Harpie Lady Sisters" during that Duel, a cutscene of "Harpie Lady Sisters" being Summoned will play afterwards.
Most of the time when she declares an attack with that monster, she announces "Harpie Lady Sisters!" followed by "Triple Scratch Attack!"
Whenever Mai Summons "Harpie's Pet Dragon", a cut-in frame of Mai will briefly appear, and she'll announce "Allow me to introduce the ever-faithful servant of Harpie Lady!" followed by "Harpie's Pet Dragon!"
Most of the when she declares an attack with that monster, she announces "Harpie's Pet Dragon, attack!" followed by "Fearsome Fire Blast!"
Whenever Mai Summons "Peacock", she announces "I Summon Peacock!"
Most of the time when she declares an attack with that monster, she announces "Peacock, attack!"
These dialogue options are a reference to Mai Valentine's Japanese name, "Kujaku Mai" (孔雀舞), since "kujaku" is the Japanese word for "peacock".
Whenever Mai activates "Amazoness Spellcaster", she announces "I activate the Spell Card - Amazoness Spellcaster!"
Whenever Mai activates "Dramatic Rescue", she announces "I activate my face-down Trap! Dramatic Rescue!" followed by "By returning 1 monster to my hand, I can bring out a different monster from my hand!"
Whenever Mai activates "Elegant Egotist", she announces "If you're already struggling with one Harpie Lady, how will you fare against more?" followed by "Elegant Egotist!"
Whenever Mai activates "Mirror Wall", she announces "My Continuous Trap - Mirror Wall!"
Whenever Mai activates "Shadow of Eyes", she announces "I activate Shadow of Eyes!" followed by "It exudes an irresistible force that lures even the mightiest monsters into battle!"
Whenever Mai activates "Triangle Ecstasy Spark", she announces "The Spell Card, Triangle Ecstasy Spark!" followed by "My Harpie Lady Sisters will show you why they're three ladies you should never mess with!"
↑ 1.01.1In-game description: You can look at the card on the very top of your Deck at any time.
↑ 2.02.12.22.32.42.5With this reward unlocked, the maximum number of Decklists that can be assigned to Mai Valentine is increased by 1.