The unnamed Genius Girl is a character from Mario + Rabbids: Kingdom Battle. She only appears during the introductory cutscene 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.
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. She also appears at the end of Donkey Kong Adventure, the DLC for Mario + Rabbids: Kingdom Battle.