Mirio and Tamaki are childhood friends and have spent most of their lives together, inspiring and supporting each other. Mirio is one of the reasons why Tamaki grew up a hero and they both have a strong bond and refer to each other by their first name, which indicates their level of closeness and respect. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.