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Professor Robert Callaghan was a professor at the San Fransokyo Institute of Technology and is the father of Abigail Callaghan. Also known as his alter ego Yokai, Callaghan served as the main antagonist for the superhero team Big Hero 6 in the movie of the same name.
Once a renowned scientist, Robert Callaghan was an aspiring man with the ambitious hope of using his research to make the world a better place. He was recognized for his work in the fields of robotics, and was the creator of Callaghan's Laws of Robotics. Later in his life, Callaghan became an instructor and the head professor at San Fransokyo Institute of Technology to mentor the new generation of prodigies. One of these students, Tadashi Hamada, admired him greatly. Though he was passionate about his work, his daughter Abigail was the light of his life. Callaghan was shown to support Abigail up to adulthood, when she would become the test pilot for Krei Tech Industries under Alistair Krei. One day, Krei founded Project "Silent Sparrow", an experimental portal design meant to revolutionize teleportation technology. However, during the experimental test run, the portal experienced a malfunction and was forcibly shut down, trapping Abigail in the space between the portals. Callaghan, who was present during the test, snapped and in a fury tried to assault Krei, blaming him for losing Abigail. The event catalyzed a transformation in Callaghan—he began to descend into madness, growing more unstable and obsessed with revenge towards Krei. Ultimately, this would lead to Callaghan discarding any semblance of his former self and the birth of the villainous entity known as Yokai.
- Yokai - Yokai is the Japanese word for "spirit" or "phantom". Individually, the kanji used for the word is "bewitching" and "mystery". The term Yōkai can also refer to a class of supernatural spirits and monsters in Japanese folklore.
- Yokai is the only "super" character in the film who has no Marvel Comics counterpart, although he does bear a resemblance to Lord Deathstrike.
- Despite being given the name "Yokai" in marketing, Callaghan's Yokai identity is never referred to by name. Although he is the only one to have a codename for his alter-ego, as the other superhero characters have nicknames for both their referred name and codename.