Yu Kamimura (Underworld)

The underworld counterpart of Yu Kamimura (上村 優 Kamimura Yū) is a mysterious young woman who lives in the Underworld, appearing in the film Kamen Rider × Kamen Rider Wizard & Fourze: Movie War Ultimatum. She is the alter-ego of Young Masked Belle Poitrine (美少女仮面ポワトリン Bishōjo Kamen Powatorin) and the Underworld counterpart to Donut Shop Hungry Manager.

History
Yu Kamimura is a young woman who apparently lives a mundane, boring life as an office lady. When her real life counterpart has been kidnapped by the Akumaizers. They convince Yu to willingly take part in their Infinite Monster Plant plan by telling her that in her Underworld, she can be whatever she wants. But, still disagreeing with their nefarious plans, Xatan cast a spell on her, causing Yu to be brainwashed into liking her new life and in her Underworld is immersed in a repeating fantasy where every day is her birthday and she's the beloved magical heroine Poitrine. In return, the Akumaizers use their machine to pull the monsters Poitrine fights into the real world for their Monster Army.

However, this changes when Haruto Souma enters Yu's Underworld to free her and is likewise caught in the time loop (as are Shunpei Nara and Rinko Daimon, who stumbled into the effect radius of Wizard's Engage Ring). Yu is immediately smitten with Haruto, affectionately calling him "Haru-chan" and trying to convince him to stay in her Underworld and be happy with her, even attempting to marry him. However, Haruto points out that four other people (pre-teen Gates who are also part of the Infinite Monster Plant) are also trapped in the loop, and her staying here makes them prisoners. Eventually Yu realizes Haruto is right, and as Poitrine helps Wizard fight off Gahra with their magic.

However, the other Akumaizers appear and kidnap Yu, refusing to let Haruto break up their plan. Thankfully reinforcements arrive in the form of the future versions of Gentaro Kisaragi, Ryusei Sakuta, and Nadeshiko Misaki, who used an unexplained Fourze Engage Ring to enter Yu's Underworld. They're further aided by the spirits of Shotaro Hidari & Philip, Ryu Terui, Eiji Hino, and Shintaro Goto, who give special rings to the four children which summon copies of Double, Accel, OOO and Birth to aid their fellow Riders. After being rescued from the Akumaizers' battle truck, Yu uses the last of her magical power to create the Special Rush Ring for Haruto, which allows him to transform into Special Rush for the final battle with Xatan.

Back in the real world, when Yu wakes up from the Infinite Monster Plant, "she" is revealed to be the Manager of Donut Shop Hungry; Yu Kamimura is his real name, and the beautiful young woman from before was how he had looked like when he was younger, having been often mistaken for a girl. Haruto is simultaneously shocked and dismayed by this turn of events, apparently having been attracted to Yu during their time together.

Ability
She is able to become Poitrine by using the Orgel Pendant (オルゴールペンダント) and shouting out "Cosmo Magic Metamorphose!" (コスモマジック・メタモルフォーゼ！). As Poitrine, she is able to use magic and use the Belle Sabre (ベルサーベル). Her common attack is to turn opponents into plush dolls.