They both have a strong bond and refer to each other by their first name, which indicates their level of closeness and respect. Mirio and Tamaki are childhood friends and have spent most of their lives together, inspiring and supporting each other. Despite the warning, Mirio did not give up on his desire, and his father supported him at all times, in spite of his former views.
When Mirio expressed his desire to become a hero, his father warned him that his family's Quirk made it difficult for them to become Heroes, which is why his father gave up on becoming one. Mirio has an excellent relationship with his father.
