Hiiragi seems to recognize Mafuyu after encountering him with Ritsuka and raises the matter of the guitar on his back and whether it is Yuki's. Hiiragi states that he thinks of Mafuyu as a younger brother and that they always used to get into small fights when they were younger. He knew Mafuyu was fatherless and considered him to be an incomplete half with Yuki where together they filled each others missing pieces. Hiiragi describes Mafuyu as submissive, introverted and reserved. Hiiragi feels like he is treated poorly by Mafuyu, but loves Mafuyu a lot.
Hiiragi knows the whole story of Yuki's suicide and was also aware of his relationship with Mafuyu but never mentioned anything about it. He thought they were both essential to one another. Whilst remembering him, Hiiragi described Yuki as flashy, dominant and moody and remembers how close he was emotionally to Mafuyu. Mentioned by Shizusumi later on, Hiiragi supposedly had a love interest for Yuki and even idolized him. Hiiragi ended up carrying a feeling of loneliness inside of him by trying to write a song for him.
Hiiragi and Ritsuka perform together when Hiiragi's band needed a guitarist. They often end up arguing about little things and yell at each other as Yagi is sitting silently by the drums looking at the two fighting.
Ever since Yuki and Mafuyu hung out together during childhood, so did Hiiragi and Shizusumi. Later in the manga, Hiiragi confesses his love and romantic feelings to Shizumi.