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 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.
Ever since Yuki and Mafuyu hung out together during childhood, so did Hiiragi and Yagi.