Genius Girl

K.C. or Kassidy Vale is a character from Mario + Rabbids: Kingdom Battle. She only appears during the introductory cutscene an the ending of the DLC of the game, where it is shown that she is the inventor of the SupaMerge, the device that sets the events of Mario + Rabbids: Kingdom Battle in motion. Judging by her room, she is a fan of the Mario series.

History
She invented the SupaMerge attempting to solve an energy crisis, but it had a problem with overheating. As she left to take a break, the Rabbids showed up in their Time Washing Machine. Spawny, one of the Rabbids, found the SupaMerge, and used it to fuse other Rabbids with several objects in the room, causing him to accidentally create Rabbid Peach and Rabbid Mario. Spawny then hit the Time Washing Machine, causing it to malfunction and teleport the girl's robot assistant Beep-0 and the Rabbids to the Mushroom Kingdom.

Also, in one of the screens of her computers also hint not only the SupaMerge is supposed to combine two things into one to make one to fix an energy crisis, it also shows some kind of wormhole, indicating that she was also trying to rip into other dimensions. That could be the reason why the Rabbids and Beep-0 entered the Mario universe. Indicating that the SupaMerge is capable of merging other realities.

A theorist also speculated that K.C. is the hidden villain of the game. This could be possibly true. The theorist explains that the cutscene has some lore into it, he indicated that the SupaMerge is actually a superweapon to eradicate her world and replacing it with a fixed energy crisis and the MegaBug is part of the plan.

Appearance
She has blonde hair with pink tips and a side-cut on her right side.

Her outfit kind of resemble to Mario (as she is a fan of the franchise),blue overalls and red shirt (although she is wearing a jacket), but the right side of the button seems to be not buttoned to her blue overalls, she uses a red jacket with one pink stripe on both of her sleeves and tucks her jacket into her overalls, and finally her white t-shirt or tank top has a super star in the middle of the shirt.

It is shown at the beginning that she never has been seen with her face to the audience, except when she reveals herself at the ending after the credits of the DLC of Mario + Rabbids Kingdom Battle: Donkey Kong Adventure.

Trivia
She is the inventor of the SupaMerge and could be the cause of the MegaBug's appearance in the game.

She intended either to fix an energy crisis or to rip into other dimensions (as explained by the theorist).

She seems to get frustrated easily when her inventions malfunction as shown when she kicked her vacuum cleaner, indicating she might be hot-tempered like Beep-0. Even though she did create Beep-0.

She has no history in game or has any relatives as shown so far.

Ubisoft is purposely keeping her a mystery among to fans as the theorist explained he had a conversation with Ubisoft creative director, Davide Soliani about her.

It's unknown if she will appear in Mario + Rabbids Sparks of Hope; though she did appear on a poster in the background when the Mario/Rabbid crew were calling a meeting.

The theorist also explained about "other realities" in the Mario/Rabbid universe when the trailer of Sparks of Hope was released, the reason they're in space and other planets, why there are more rabbids merged with other beings, and the theorist calls it: The Merging Effect, it's likely an interdimensional space spore created by the SupaMerge and K.C. to eradicate the world by "Merged Creatures." A new world as K.C. sees fit for the entire universe. If not, the multiverse.