Yokai

Professor Robert Callaghan a.k.a Yokai is the main antagonist of Big Hero 6. He is a former university teacher presumed dead in a fire; until he later re-emerged using a vast army of microbots in his plan to take revenge on Alistair Krei, whom he blames for the supposed death of his daughter, Abigail Callaghan.

He is voiced by James Cromwell.

Big Hero 6
Professor Callaghan is a renowned roboticist in San Fransokyo, credited with the creation of Callaghan's Laws of Robotics. He first appears in Tadashi's university, the Institute of Technology, with Tadashi and his younger brother Hiro. Upon introducing himself to Hiro, Callaghan speaks that Hiro's fighting robot employs his laws, and subtly reprimands him for bot-fighting as opposed to putting his intellect to use. It is this that motivates Hiro to join Tadashi's university and to invent the microbots. Hiro's subsequent demonstration of his new microbots in order to join the university visibly impresses Callaghan, as well as Alistair Krei.

Krei expresses interest in buying Hiro's microbots, but Callaghan intervenes with Krei's offer and states his distrust of Krei, convincing Hiro not to sell his microbots (a subtle reference to his hate of Krei). Once Hiro denies to sell the microbts, Callaghan accepts Hiro into the university. After the presentation, a fire breaks out in Tadashi's showcase with Callaghan still inside. Tadashi runs inside to save Callaghan, but the building explodes, killing Tadashi and leaving Callaghan to be presumed dead.

Callaghan, as Yokai, is later found within a warehouse in a shadier part of San Fransokyo, where Hiro's microbots led him. Hiro and Baymax discover what appear to be robotic arms (highly resembling Callaghan's robotic arms in the showcase) manufacturing microbots by the dozen. Before Hiro could investigate any further, Yokai appears and bundles the microbots into a slithering wave, and attempts to kill Hiro and Baymax. Hiro catches a brief glimpse at the masked man before escaping out of a window. Hiro explains the matter to the local police station, but the cop he tries to explain the matter to is unsure whether to believe him or not, and Baymax's battery runs low, rendering his evidence almost completely unreliable.

Hiro, still devastated about the loss of his brother, and theorizes that Yokai stole his microbots, then caused the fire in Tadashi's showcase to cover his theft. Determined not to let him escape, Hiro builds Baymax a suit and installs combat into his databases, as a chip of his design. Hiro tracks down Yokai near the warehouse. Hiro sees Yokai and ducks for cover, and seeing the villain gathering massive parts from the nearby bay with the microbots. Suddenly, the rest of Tadashi's friends (GoGo Tomago, Honey Lemon, Wasabi, and Fred who Baymax had contacted earlier) drive in. Hiro desperately attempts to explain the situation, but Yokai sees them and attempts to crush them. Narrowly saved by Baymax, Hiro and the rest of the team drive off with Yokai right behind them. Yokai attempts to kill them by throwing vehicles and using his microbots as a wave to attack them. It ends with Gogo accidentally driving a car into the bay and sinking into the water. Yokai looks on, and, thinking them drowned, leaves.

Baymax saves Hiro and his friends by discarding his armor and using his buoyant balloon body to float them back up to the surface. They retreat to Fred's house, which, surprisingly, is a fancy mansion. Baymax then reveals that he scanned Yokai. Planning to do a mass scan on San Fransokyo to find out his identity, Hiro builds a new battle suit for Baymax, equipped with offensive capabilities and rocket-propelled flight, and designs suits for his friends, organizing the team known as Big Hero 6 to bring Yokai to justice. Baymax and Hiro fly above San Fransokyo and scan the city, and Yokai's information is matched to a distant, quarantined island.

Flying down to the island, the Big Hero 6 discover what appears to be a machine in progress. A video feed shows the construction of the machine, which was meant to be a massive teleportation device. Krei is seen in the feed, and a mortal accident accidentally ends up with the machine exploding while a test pilot named Abigail is launched inside as it does, leaving her to be presumed dead. This leaves Big Hero 6 to assume that Krei is the guy under the mask since the government forced him to shut down his experiment after Abigail's supposed death, and they believe Krei is using Hiro's microbots to steal his own machine back. Before the feed continues however, Yokai suddenly appears, armed with the microbots, and yet again attempts to crush the heroes. Hiro and everyone else fight Yokai, ending with Hiro and Baymax flying into Yokai and knocking off his mask.

As they got up, Yokai turned and showed his true identity as Professor Robert Callaghan. Realizing that Callaghan was the one who caused the fire that killed Tadashi, a shocked Hiro confronts him for this, only for the professor to coldly explain that Hiro's microbots saved him and that he believes Tadashi's death as his own mistake. Out of complete rage, Hiro removes Baymax's healthcare chip and orders him to kill Callaghan. Baymax goes out of control and attempts to kill the now powerless Callaghan under Hiro's orders, but the other members try to stop him, allowing Callaghan to gather up the microbots and flee. Baymax is forcefully stopped and Callaghan reclaims his mask to use the microbots to escape, much to Hiro's rage.

Hiro is very angry at his friends for interfering and flies off with Baymax, leaving the others behind on the island (although Fred tells him that his butler picked them up and flew them back in his family's helicopter). After trying so many failed attempts to gain access to his access port, Baymax asks if killing Callaghan is what Tadashi would have wanted. With Hiro entering a fit of anger and despair, Baymax shows Hiro a video compilation of Tadashi making improvements, making Hiro regain his senses and realize that killing Callaghan won't do any good. Big Hero 6 regroups and looks over the video feed again. It shows a visibly furious Callaghan storming into the building, raging at a frightened Krei about how his teleportation portal "wasn't ready" and "unsafe" and that he was using it without consent. The building was quarantined by the government, and Abigail the test pilot is revealed to be Abigail Callaghan, the professor's now supposedly dead daughter. It was then Big Hero 6 realized that Abigail's apparent death drove Callaghan to a point of vengeful anger, leading him to adopt the masked costume and manufacture the microbots to take revenge on Krei.

Big Hero 6 rushes over to Krei Tech's main building, where Krei is making a speech about the new building's opening. Callaghan, again as Yokai, suddenly intercepts, seizing Krei and dragging him into the air and directly confronting Krei about his daughter's supposed demise. Krei begs that Abigail's presumed death was an accident, but Callaghan dismisses his plea and accuses Krei and his arrogance and greed, and lack of regard for the consequences of his actions for being the cause of Abigail's death. He then tells Krei that he took everything from him when he sent Abigail into the portal and apparently disregarded her life and now he will take everything from him in retaliation. Callaghan takes the remains of the teleportation machine and with the microbots, rebuilds it over Krei Tech's building, intending to destroy it. He tells Krei that he will make him watch everything he built be destroyed, then he would kill Krei next.

Big Hero 6 arrives, and Hiro tries to reason with Callaghan to spare Krei, admitting that he understands his pain and knows the very feeling of loss. Realizing that what Hiro says may be true, Callaghan hesitates and appears to consider stopping his plan. However, when Krei cowardly pleads for mercy, the villain once again allows his disgust at Krei's lack of courage to consume him and instead opts to kill both Krei and the Big Hero 6, saying furiously that he wishes to have his daughter alive again, using the microbots to pin down each member of Big Hero 6. In a climactic final battle, Hiro has everyone work from different angles, succeeding in tricking Callaghan into sucking his microbots into the portal the machine above created.

Callaghan (without anymore working microbots) is about to receive a punch in the face by Baymax, but it turns out that the robot decides to spare the former professor. Baymax the removes the mask and crushes it, rendering the remaining microbots useless. As such, Callaghan starts to fall along with the portal, only to be saved by Baymax and Hiro. Before Callaghan can be arrested, Baymax detects life within the teleportation machine's dimension, revealing that Abigail is alive but in hypersleep, much to Callaghan's surprise. Hiro and Baymax fly into the portal and successfully manage to draw Abigail out, but at the cost of Baymax's original body.

For his crimes, Callaghan is arrested by the police, but he manages to see his daughter one last time before being driven off in a police car, and his sad facial expression implies that he feels remorse for his actions as he awaits to be given a life-sentence in prison for his crimes. What happens to him in prison is unclear.