- “I wouldn't be here if it weren't for you so, you know, thanks for not giving up on me.”
- ―Hiro to Tadashi
Hiro is Tadashi's younger brother and likely the most important person in Tadashi's life. He and Hiro had also become best friends, considering their duo as an 'exclusive club', according the Hiro and Tadashi book. They liked to keep secrets and make up rules that only the two can know. They shared a bedroom presumably since they first moved in with Aunt Cass 10 years prior to the events of the film, allowing Tadashi to always be there when Hiro needed him, academically or emotionally. Because they were raised together, no one knew Hiro's emotions and issues better than Tadashi and vice versa. Tadashi was also the one who encouraged Hiro to become interested in robotics and they are seen assembling a robot toy together in a family photo as children. Hiro's health, happiness and well-being were of utmost importance to Tadashi and he was disappointed when Hiro continued to bot-fight underground instead of trying to do something productive with his incredible intelligence. He not-so-subtly persuaded Hiro to apply at San Fransokyo Tech and pursue a career in robotics alongside him and is tremendously proud when Hiro succeeds.
Cass is Tadashi's over-worked but loving aunt. She had raised him and Hiro for a decade since their parents died, although she admits that she's not perfect. He hated making her upset and tried his best to make her proud, encouraging Hiro to do the same.
Tadashi appeared to be the only one with clear memories of his and Hiro's parents, being the older of the two. He lived his life in consideration for what they would have wanted and reminded Hiro of how disappointed they would be had they known he was participating in underground bot fights.
- “You're going to help so many people, buddy. So many!”
- ―Tadashi, after he successfully gets Baymax to work properly
Tadashi is the creator of Baymax. Before completing him, Tadashi suffered from Baymax's unintentionally erratic and violent behavior, such as quickly jabbing into Tadashi's head with his fingers, shorting electrically (creating a burn hazard) or hurting his ears with high-pitched noises. As soon as Baymax functioned properly, however, Tadashi was ecstatic, humorously running around the room excitedly before asking the robot to scan him. Tadashi was incredibly hopeful at the existence of Baymax, hoping that someday, the healthcare companion could help the sick and injured all over the world.
After Tadashi's death, Baymax takes his place as Hiro's emotional support, something that Hiro doesn't initially want, nor does Baymax understand. Attempting to make sense of Tadashi's absence is also one of the first times that Baymax deviates from his normal functionality. At one point, he calls Tadashi's name while looking at Tadashi's belongings, wondering where he is and when he will come back, something unusual for him at this point. He did not understand that Tadashi could be dead, as he had previously scanned him and calculated that Tadashi was in excellent health.
Wasabi, Go Go, Honey Lemon, and Fred
Wasabi, Go Go, Honey Lemon and Fred were Tadashi's classmates and closest friends. They were shown to have a close bond and were regularly seen with one another outside of school. After Tadashi's death, the remaining group members took it upon themselves to watch over Hiro and eventually welcome him into their circle of friends. Not only that, but they also carried out Tadashi's hopes and dreams of helping the world by remaining members of the Big Hero 6 superhero team.
Professor Callaghan was Tadashi's professor and mentor at the San Fransokyo Institute of Technology. The two shared a unique relationship amongst the institute's crowd, as they showed a high level of mutual respect, bordering the lines of a father/son relationship. On Tadashi's side, the young prodigy admired Callaghan greatly for his aspiring ways and innovations in the world of science and technology. Like his mentor, Tadashi was shown to be an inspiring and wise innovator, especially towards Hiro and his friends. Judging by their interactions and similar outlooks on science and intellect, it can be implied that Tadashi learned such traits from Callaghan.
The strength of Tadashi's admiration for Callaghan was truly established moments before the former's death, as the young man was tragically killed trying to save his mentor from a huge and deadly fire. Unfortunately, the sacrifice was ultimately for nothing, as Callaghan was the one responsible for starting the fire, which he did in order to cover the tracks of his crime of stealing Hiro's microbots. When told Tadashi died on his account, Callaghan showed no remorse for causing the tragedy, callously dismissing Tadashi's death as his own mistake. With this, while Tadashi obviously admired Callaghan, it is unknown whether or not Callaghan truly cared about Tadashi. However, the very least that can be said is that Callaghan did not intend to outright kill Tadashi when he started the fire to cover his theft of the microbots.