Yokai, as portrayed in the "Big Hero 6" manga series (titled "Baymax" in Japan), is an adaptation of the film antagonist of the same name. His true identity is Robert Callaghan.
Robert Callaghan was once an aspiring scientist; however, he found himself too distracted by his work to miss the vital moments of his life, from his daughter Abigail's birth to the death of his wife. After Abigail disappeared, he became Yokai as revenge by using microbots to take anyone in his way.
On the surface, Callaghan is a firm and efficient professor, equable and has a strong sense of academic integrity. He apparently looks out for his students' well-being, so much that he defends Hiro when the boy was accused of harming the student body with his invention, and was considerate of the individual feelings of his students. Calculating and observant, Callaghan is quite astute thus he is not influenced by assumption as easily as others.
However, the scholarly personality is but a façade; his true personality is that of a ruthless mastermind, consumed by his desires to the point that he displays sociopathic tendencies to the students he once supported.
- Yokai from the manga and the film are quite different in personality traits, as the manga depicts him being a ruthless villain, instead of being antagonistic due to circumstance in the film. Also, his personality as Robert Callaghan is far less fatherly and more serious, possibly to make his darker nature more apparent.