U.A. High School
Tamaki has known Mirio since childhood when they first met in the third grade, saying Mirio and him had been friends ever since, and are on a first name basis with each other, indicating a level of closeness. Tamaki's hero name, "Suneater", was also given to him by Mirio.
Tamaki and Mirio first met when Tamaki transferred to Mirio's school, when Mirio asked if Tamaki had been trying to talk about heroes in his stuttered self-introduction, and if so, who his favorite hero happened to be.
He was surprised at the sight of Mirio's sadden state after learning of Eri's situation, and Mirio showing visible concern after learning about the injury Tamaki received, asking very swiftly if he was okay.
Tamaki admires Mirio. He praised the techniques Mirio achieved from the hardships he went through. He told Eijiro he shines like the sun similarly to Mirio, something Tamaki himself feels a lack of confidence to achieve.
Mirio also admires Tamaki to some unknown degree, as seen in a flashback, Mirio commented he was able to give it his all because of his presence. This reciprocating admiration and his own admiration for Mirio's personality is a strong motivation for Tamaki to gather his assurance and courage to fight. Mirio also reassures Tamaki when he feels insecure, such as when Tamaki had begun to go a tangent about how the heroes would not be able to save Eri or even themselves, by interjecting his rambling with words of encouragement, both reassuring Tamaki and spurring him into action.
When Mirio got shot by Shin Nemoto during his fight against Overhaul with the Quirk-Destroying Drug bullet, Tamaki woke himself up out of unconsciousness and felt something bad happen to Mirio. It is implied that right after that, Tamaki went to search for him, and finally carried Mirio out of the building.
When bringing up the topic of Eri's Quirk, Tamaki placed his hand on Mirio and mentioned Mirio's Quirk possibly returning and said his future of a great hero might be real. Mirio thanked him for his belief and smiled at his friend.
While they are not on a first name basis, the two have known each other during their time at U.A., and thus appear to be somewhat close.
Tamaki treats Nejire's talkative attitude casually with tolerance and would attempt to keep that personality of hers in check when required. Nejire finds Tamaki's anxious personality weird, calling it 'chicken-hearted', which becomes her nickname for him.
Tamaki is initially somewhat annoyed by Eijiro's optimism, but at the same time compliments him for it, and later compared him to Mirio. Eijiro has a lot of respect for Tamaki, due to him being his senior. As seen when he refused to let go of the criminal who had previously shot Tamaki. He is also shown to be thankful for his senpai because it's thanks to him if he could join Fat Gum team.
Fat Gum scouted Tamaki to work for his office. Fat Gum sees a lot of promise in Tamaki but constantly teases and demands Tamaki to make some change to his cruddy mentality.
Tamaki does not like Taishiro's teasing, claiming Taishiro only scouted him to torment him in what Tamaki calls "workplace harassment."
Since Taishiro is shown to be a man who eats a lot, this seems to complement the requirements of Tamaki's Quirk.