Mifu looks identical to Fuu2, as they both have similar body shapes and having shaped numbers as traits in their physical appearances, but unlike Fuu2, she is pink, has Katie's face and hairstyle, and there's a dark rose pink heart shape seen on the bottom left. Her ponytail was shaped into a "3".
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In Yo-kai Watch 3: Sukiyaki, after the player defeats Injaneno in Nate's story, the player has to go to Nate's house and talk to Hidabat, to start the request "That is a World that exists Somewhere" (Japanese: それはどこかにある世界,"Sore wa doko ka ni aru Sekai").
Hidabat will hand him a package he received which contains issue 23 of the Nu Magazine that comes with a special Nu Nr. 23 Coin.
After trying out the coin on the Crank-a-kai, Nate ends up finding himself transported into what turns out to be a different world, in which he takes on the form of Fuu2. He soon learns that this is a parallel world to his own, and that this world's equivalent of Katie Forester was the one to free Whisper from the Crank-a-kai and receive a Yo-kai Watch, instead of this world's equivalent of himself.
Fuu2 does eventually make it back home to his own world, but is then soon approached by Mifu, who reveals herself to be the parallel world's Katie, having come to his world the same way he had come to hers.
She gives him her medal and the two decide to continue to visit each other from now on.
In Version 3.0 of the game, the request "Girl Fight" (Japanese: 女の戦い,"Onna no Tatakai"), which continues this storyline, was added, in which Nate visits the other Katie's world again, and attends to her own visit of Yopple Inc. and meets her world's equivalents of Hailey Anne and Usapyon.
In Version 4.0 of the game, the request "The Three Yo-kai Watches" (Japanese: 3つの妖怪ウォッチ,Mitsu no Yōkai Wotchi) was added, which continues this storyline even further.
Mifu tells Nate that she has become unable to return to her own world, after having lost a fight with mysterious black Yo-kai that have began appearing in her world.
With the help of both Lord Enma and Ankoku-shin Enma, the latter being from a parallel world himself, both Nate and Mifu make it back to her world. Once there, Nate realizes the black Yo-kai she's been fighting are actually the Gogogo Family.
As they try to track down their leader, their are confronted by this world's Hailey, who after losing a battle with the Gogogo Family has been brainwashed into becoming their ally.
They manage to save her, and now, with the power of 3 Yo-kai Watches on their side, they confront this world's Gogogo Godfather and defeat him. Afterwards, Nate and Katie share a heartfelt goodbye, uncertain if their worlds can ever connect again after this.
However, some time later Katie simply checks if she can still summon Nate as Fuu2, since she still has his medal, and finds it still works. Fuu2 is happy to see her again, but also upset because his own world's Katie had just come over to visit him, which causes the other Katie to tease him, suggesting he prefers his own world's Katie over her.