Hakkai and Mitsuya first met as children when Mitsuya stopped Hakkai from beating up another child. From then on, Mitsuya instilled his values onto Hakkai, believing that Hakkai was protecting his sister from abuse. Hakkai was the vice-captain for Mitsuya's second division and showed great admiration for his captain, even using Mitsuya's picture as his lock screen. He sees Mitsuya as a real older brother, describing their relationship as "unfortunately close".
Mitsuya values Hakkai as well; to help Hakkai, he negotiated with his brother. He shows up at the chapel to aid Hakkai in facing his brother. Additionally, Mitsuya's conduct at the Christmas Conflict further impressed Hakkai, leading him to initially believe that his sister was in love with Mitsuya. After Tenjiku's attack on Mitsuya, Hakkai considered resorting to fighting unfairly. In the present, he works as an internationally successful model, the same field in which Mitsuya works.
Mitsuya is the person Draken has known the longest of Toman's members. One night, Draken was out on a walk and eating Karaage Don when he sees a dejected Mitsuya spray painting a huge dragon design on a wall. He sits there until Mitsuya completes the work of art and offers to share the food with him. Mitsuya finishes the food and leaves nothing for Draken, so in exchange, Draken claims the dragon design as his own.
Later, Draken invites Mitsuya to the brothel where he lives. They play cards with the prostitutes and the two bond with each other. Later, as Draken walks Mitsuya home, he asks if Mitsuya likes his mother's cooking, to which he receives the reply that her cooking is bad and he would have his own. Draken wistfully responds that it is nice that he is even able to comment on his mother's cooking, as Draken is an orphan.
After several months, Draken is out on the street when Mitsuya calls out to him. They both exclaim in surprise when they see that both of them had gotten the dragon design that Mitsuya painted back then tattooed on their temples. Since then, they have been inseparable friends.
Yuzuha and Mitsuya seem to have a neutral treatment before the Christmas conflict. Although Yuzuha seems to feel a little jealous of Mitsuya as an older sister, because Hakkai prefer to have Mitsuya as a wallpaper on her phone instead of her (as Yuzuha, who has Hakkai as the wallpaper on her phone).
Throughout the Black Dragon arc, Mitsuya has knowledge of how abusive Taiju Shiba is. And he decides to help Hakkai prioritize Yuzuha's well-being by allowing Hakkai to leave Toman on the condition that Taiju will not hurt Yuzuha further.
Later during the Christmas Conflict, after Hakkai confessed that it was him his sister was actually protecting from Taiju, Mitsuya apologized to Yuzuha, due to the strong expectations he had instilled in Hakkai on how to be a brother and that It led him to lie. Considering then that he wasn't as good a big brother to Hakkai as he thought. Mitsuya concludes that apology by expressing to Yuzuha that he respected her.
Yuzuha sees him as a brother and he seems to see her as a little sister.