As much as I have been trying to avoid spoilers until I can see the film for myself, the whole concept of Yokai's identity and Tadashi's "murder?" going to many suspects (3 at the most) ought to be handled in murder-mystery fashion, for instance like Clue or Murder, She Wrote, where almost everyone has a motivation to either kill/kidnap Tadashi or steal the microbots to become Yokai:
Tadashi Hamada. Motivation: Brainwashed or turned renegade by free will to do the real villain's bidding despite his niceness.
Alastair Krei. Motivation: To get revenge on Hiro for rejecting his offer with the microbots.
Robert Callaghan. Motivation: None that I can think of, but I'll wait until the movie comes out. When in doubt, it could be for personal gain or the same reason as Tadashi.
A robot: Motivation: To follow the orders of the real villain.