Mr. Futaki

Saigusa Haruka's and Futaki Kanata's common father.

Later in Haruka's route there was revealed that Haruka has not only her own older twin sister, but also the other father whose name is Saigusa Shou, and even Kanata and Haruka were born to the same mother but to both exact different fathers(via heteropaternal superfecundation for explanation).