Professor Robert Callaghan (also known as Yokai) is the main antagonist of Disney's 2014 animated feature film Big Hero 6. He is the former head robotics professor at the San Fransokyo Institute of Technology and mentor to Tadashi Hamada. After suffering a personal tragedy, he became a silent, kabuki-masked supervillain bent on exacting vengeance, with an army of Microbots at his command.