Tadashi is Hiro's older brother and was the most important person in Hiro's life, even after his death. Because their parents died when he was at a young age, Hiro has always looked to his older brother for guidance and advice. They were best friends and even had a set of rules they came up with that only they knew (according to the book Hiro and Tadashi). They've shared a bedroom since they moved into their Aunt Cass's cafe but don't seem to mind and enjoy each other's company. Tadashi has always been there for Hiro whenever he needed him and the two balance each other well, as Hiro is more creative with his inventions while Tadashi is more practical and by the book as shown in Hiro and Tadashi. Tadashi sees the potential in Hiro and feels it is wasted when, after Hiro graduates from high school, he chooses to engage in a career in botfighting rather than continue his education and use his knowledge to improve the lives of others. Because Hiro is cut off from other people and because Tadashi has been with Hiro since the moment he was born and practically raised him, Tadashi understands Hiro the best out of everyone and was even the one to interest Hiro in robotics. He is also more able to persuade Hiro to do things, as shown when he manages to persuade Hiro to apply to San Fransokyo Institute of technology. He is kind and caring towards Hiro, but is also capable of being stern, demonstrated when he lectured Hiro about how he would get into trouble eventually for his bot fighting obsession and wanted him to stop. He was also goofy, as shown when he grabbed Hiro upside down to shake some ideas into his head. Nonetheless, he was encouraging of Hiro's intelligence and pursuit of robotics and was always there to protect him, as shown when he saved Hiro from Mr. Yama and his henchmen.
Tadashi's death had the greatest impact on Hiro, who fell into depression and refused to leave his room or eat after the incident. He also refused to attend the San Fransokyo Institute of Technology, despite his initial excitement to join. The death of his older brother also hardened Hiro's heart, and led him down a path of murderous vengeance, as seen when the boy manipulated Baymax's programming in hopes of killing Yokai, only to be stopped by his own friends.
When he tried to pursue the path of vengeance again, Baymax refused, as the latter knew this would not help Hiro and wouldn't be what Tadashi wanted. Hiro then watches clips of Tadashi building Baymax on the robot's video monitor and realizes Tadashi's goal was to help people and that he should do the same. This inspires Hiro to seek justice rather than revenge on Callaghan, make up with his friends, and continues to do good for the world by becoming a hero.
Cass Hamada is Hiro and Tadashi's aunt. Hiro tends to stress her out and cause her to "stress eat" due to his illegal bot fighting, though she loves him nonetheless and vice versa. She is very supportive and proud of his abilities and was overjoyed when his Microbot exhibition won him an invitation to attend the San Fransokyo Institute of Technology.
Like Hiro, she also mourned the passing of Tadashi. She would bring food to Hiro after Tadashi's death as Hiro fell into a severe state of depression and refused to leave his room for two weeks. She tried to encourage him to go to college because Tadashi would've wanted it and she wanted him to recover, but he remained silent and she decided to give him space. When Hiro left his room for the first time in two weeks and he lied about going to register for classes (he was really going to catch up to Baymax), she was overjoyed and hugged him.
Overall, the two have a positive relationship and are close, especially since now they are the only known close relatives each other has. However, it is interesting to note that Hiro did not want to tell her about Baymax and that he seems to be keeping his superhero persona a secret from her. This is most likely because she has been shown to worry about Hiro and disapproves of him doing dangerous activities (such as bot fighting) and after Tadashi's death, she might forbid him to continue being a superhero as it would put his life at risk.
Their relationship started out basic. Hiro was impressed with Baymax when first introduced, but when they reunited, he was less than thrilled as Baymax served as a reminder of Tadashi's death. However, because Hiro unintentionally summoned Baymax when the former expressed pain, the robot viewed Hiro as his patient, becoming solely devoted to him and unable to dismiss himself until the boy was satisfied with his care, vocally expressing that fact. This led Baymax to carry out every small order given by Hiro, no matter how literal or sarcastic, in hopes of improving the boy's health, as a result of his programming.
As time went on, however, Baymax's treatment towards Hiro began to change. As the robot began learning and understanding the basics of human emotion, a personal bond with Hiro began to form. His dedication towards his "patient" was no longer due to programming alone, but also because of a mutual love. In turn, Hiro grew to care for Baymax, who served as his ultimate form of comfort, a loving reminder of Tadashi and a nurturing companion. This is powerfully showcased when the duo found themselves trapped within Yokai's portal while trying to save Abigail. While Baymax was willing to sacrifice himself, Hiro strongly refused, as he didn't have the strength to lose him too. Nevertheless, he fought through his emotions using Baymax's lesson of overcoming loss, but not forgetting, allowing him to bid farewell to his companion, in addition to expressing his love for the first time through a hug.
Following these events and Baymax's eventual revival, the duo remains as close as ever, forming a brotherly relationship and spending their days beside one another through all events, including their mission to protect the world in Tadashi's honor.
Hiro was first introduced to the rest of his team (including Fred, Go Go Tomago, Wasabi and Honey Lemon) when he was tricked by Tadashi into visiting his college. He got to meet each of Tadashi's best friends and understand their individual quirks. He was impressed with all their inventions and was surprised by Fred with his mascot costume. Hiro then proclaimed his desire to apply at the institute and immediately began working to develop Microbots. During the time of and before the convention, he had become good friends with everyone.
After the death of Tadashi, the team members took Hiro under their wings, comforting him, supporting him, taking care of him and protecting him throughout the film in place of Tadashi, creating a close bond. This continued even after Baymax's undesired abandonment near the end of the film, as his friends helped him cope with yet another tragic loss. Honey and Fred serve as enthusiastic supporters, Wasabi often watches out for Hiro's health and physical well-being and Go Go is arguably the closest to the boy, eventually taking on the role of a protective older sibling figure to him.
Being a word-renowned scientist and personal mentor of Tadashi, Hiro initially looked up to Professor Callaghan, showing great respect and admiration towards him. Impressed by Hiro's knowledge, Callaghan was able to slyly convince the boy genius to give up botfighting to join the Institute to "challenge himself." It is possible he did this knowing of Hiro's intelligence and wanted him to apply to the school in hopes of stealing one of his inventions (though this is up for debate). Hiro impressed Callaghan at the convention with his Microbots and Callaghan warned him against accepting Krei's offer to buy his creation, subtly manipulating him to say no. When Callaghan was "trapped" in the burning building, Tadashi went in to save him, losing his life in the process. When Hiro discovered Callaghan had started the fire to steal the Microbots and thus indirectly killed Tadashi (showing no concern for Tadashi's life or the fact that he tried to save him), he was enraged and attempted to kill the professor as revenge; he was only stopped by his team's intervention. However, after Baymax calmed him down, Hiro realized his beloved brother wanted to help people, not hurt them, thus realizing he needed to bring Callaghan to justice without harm.
Hiro soon learned from his teammates that Callaghan had suffered the loss of his daughter Abigail (thanks to Krei's misuse of the portal technology that led to her supposed death), and that Callaghan only stole the microbots as part of his true plan to get revenge on Krei. When Hiro confronted Callaghan once again, he tried to reason with him, admitting that he does feel sorry for him and understands the pain of loss, and wanted to give the same support his teammates provided him with. However, a bitter Callaghan, while he did show a hint of regret, refused to back down and battled the boy genius, in hopes of killing him. After destroying the Microbots, thus leaving Callaghan powerless, Hiro made the ultimate decision to spare the former professor's life, having set aside his thirst for vengeance in exchange for the moral codes of Tadashi and Baymax. Not only that, he also made the choice to risk his own life to successfully save Callaghan's daughter, after discovering she was still alive within the portal. Though Callaghan showed no forward gratitude towards the hero, he met his defeat through imprisonment, and presumably permanent separation from his only loved one, finally receiving punishment for all the damage he's done, as well as granting Hiro closure. However, Callaghan's expression at the end implies that he feels remorse for what he had done, ultimately realizing that what Hiro said is right about the emptiness and consequences of revenge.
The emotional core of Big Hero 6 is driven by four key characters, their arcs, and the relationships they share with one another, being Hiro, Tadashi, Baymax, and Callaghan. As such, Callaghan serves as a omen of what Hiro would have become if he allowed his pain to consume him. However, unlike Callaghan, Hiro had friends and family who supported him through the ordeal and reminded him justice, not vengeance, is the answer.
Later, when Fred assumes that Krei was Yokai, Hiro dismissed it, saying that he's too high profile, possibly implying that he is highly respectful of Krei's public image. When Hiro went to an abandoned Krei lab to track down Yokai, he initially grew to believe Krei was Yokai but was proven wrong after taking Yokai's mask and learning it was Callaghan. He also learned that Krei's teleportation experiment resulted in an accident that presumably killed Abigail which eventually turned Callaghan into Yokai. Despite finding this out, Hiro didn't seem to hold a grudge against Krei for indirectly creating Yokai as he tried his best to protect him from the masked villain. This could be because he knew it was an accident or because he wanted to stop Yokai's plan for vengeance. Krei was ecstatic when the Big Hero 6 team came to rescue him and even openly stated his love of Baymax, apparently growing a true liking for the team following these events.
When Hiro goes to save Abigail, Krei objects, likely showing concern for the boy's safety. However, Hiro assures him by telling him the same words that Tadashi had told him "Someone has to help". Krei seems to have learned his lesson and even built a showcase hall in Tadashi's honor, possibly as a way to make amends since he indirectly created Yokai and therefore was linked to Tadashi's death. Hiro accepted the honor on behalf of his late brother.
While the two don't have much interaction, it can be presumed Hiro holds no hard feelings against Krei and Krei is thankful for everything Big Hero 6 has done for him.