Kumagorou Mikadono is a is character of Kono Naka ni Hitori, Imouto ga Iru!. His death at the start of the series is what started the chains of the events.
Kumagorou was the previous president of the Mikadono Group. He is also a friend of Konoe's father. He apparently fathered an illegitimate daughter (which is latter revealed as Miyabi Kannagi) but his wife Kanoko does not want anything to do with the child, thus Shougo has no leads as to who the girl's identity is.
Kumagourou's last wish before death was to be able to see his son become the head of the Mikadono group. It was not revealed if he did or not