Yokai's Mask was a mask that was worn by robotics expert Professor Robert Callaghan while under the supervillain alias Yokai. It was a white Japanese Noh theater mask with red markings and yellow eyes, giving it a sinister appearance. It was equipped with the neural transmitter originally created by Hiro Hamada, giving Yokai the ability to control the Microbots. Without the mask, Yokai would lose control of the Microbots.
Hiro first saw the mask when Baymax took him to a warehouse thanks to the last known Microbot in existence. Seeing a masked man controlling far more Microbots, Hiro tried to report this to the police but his story wasn't believed. Hiro thought that this man had caused the fire that killed his brother Tadashi and Professor Callaghan to steal his Microbots, so he set off to apprehend the man himself with help of Tadashi's friends.
During a scuffle between Yokai and the group of friends at what used to be a testing facility owned by Krei Tech on Akuma Island, Hiro managed to remove the mask, and Yokai was revealed to be none other than Callaghan himself. Callaghan demanded Hiro give him back the mask, to which Hiro responded by corrupting Baymax and ordering him to terminate Callaghan. As rest of the team attempted to stop Baymax from committing murder, the professor retrieved the mask and escaped with the Microbots.
In the final battle against Yokai, the team initially aimed for the mask, but Callaghan proved to be unstoppable due to his control of the Microbots, so Hiro had to come up with a plan to strip Callaghan of his power. After the Microbots were eliminated, the mask was finally destroyed by Hiro and Baymax, thus causing Callaghan to permanently lose control of the Microbots.