Tadashi Hamada was Hiro Hamada's older brother, the eldest nephew of Cass Hamada, and a genius inventor from Big Hero 6. He took a lot of steps to build Baymax, according to Baymax's flashback.
Tadashi was born as the first-born son of Mr. and Mrs. Hamada, who had a second son named Hiro a few years later. When Hiro was around three years old, Mr. and Mrs. Hamada died, resulting in Tadashi and Hiro being taken in by their aunt Cass.
Big Hero 6
Ten years later, Tadashi comes to Hiro's aid after he is pursued by the goons of Mr. Yama, whom Hiro beat in a bot fight. As the brothers drive away on Tadashi's motorbike, the former berates Hiro for taking part in bot fights as they are illegal. Tadashi, Hiro, Mr. Yama and the goons end up being arrested by the police, with Hiro getting his own cell (due to being a minor) and Tadashi having to share a cell with Mr. Yama and his goons. That night, the two are bailed out by Cass, who scolds them as she drives them back to their home at The Lucky Cat Cafe.
Despite what has happened, Hiro wishes to attend a bot fight taking place across town, to which Tadashi offers him a lift. However, Tadashi instead takes Hiro to his university, The San Fransokio Institute of Technology where they meet Tadashi's friends Wasabi, Go Go Tomago, Honey Lemon and Fred. Tadashi then takes Hiro to his lab and shows him his invention; an inflatable healthcare robot named Baymax. Hiro then meets his idol and Tadashi's mentor Robert Callaghan, who convinces Hiro to join the university. In order to be enrolled, Hiro has to take part in a science fair to convince Callaghan to let him join.
Back home, when Hiro struggles to think of an invention, Tadashi tells him to look at a different angle. This convinces Hiro to create Microbots, which impress Callaghan enough to let him join the university. Afterwards, Cass announces a celebratory dinner at the cafe. Tadashi tells her that he and Hiro will catch up with her and the others, so the brothers can have alone time. Tadashi congratulates Hiro on being enrolled moments before noticing fire coming from the Institute's showcase hall. After hearing from a survivor that Callaghan is still inside, Tadashi runs inside the burning building to try and save his mentor, telling Hiro that someone has to help. Hiro attempts to go in after his brother, only to be knocked out by an explosion, which kills Tadashi. A vigil was held outside the convention. Weeks later, Baymax says how Tadashi loves Hiro so much before he died and how he built Baymax.
Hiro later learned that the fire wasn't an accident and that Callaghan had started the fire to steal the Microbots and blamed Tadashi for his own death, enraging Hiro, until Baymax managed to calm him down by showing him videos of Tadashi testing him until he was right and expressed excitement of showing Baymax to Hiro.
At the film's end, Hiro along with Big Hero 6 avenge the death of Tadashi by defeating Callaghan and turning him over to the police for his crimes. In the closing credits, we then see that some time afterwards, a new building is built in Tadashi's honor, "Tadashi Hamada Hall", likely meant to be a replacement to the Technical Institute's showcase hall where Tadashi died.
Tadashi and his younger brother Hiro
Tadashi shows Hiro Baymax
Tadashi with Aunt Cass, Go Go Tomago, Honey Lemon, Fred and Wasabi
Tadashi is similar to Sitka as both are the older brothers of the protagonists (Hiro and Kenai) and both died trying to save someone from a threat (Tadashi dies trying to save Callaghan from a fire whilst Sitka dies saving Kenai and his other brother Denahi from a bear).
In the early drafts of Big Hero 6, Tadashi, along with Hiro, was originally going to be Cass's son rather than her nephew.