Genbu Kururugi is the former prime minister of Japan before he was killed by his son, Suzaku. In his flashbacks, Suzaku remembered how angry he was with his father, because he wanted to wage a war. Suzaku stabbed him out of anger, and he's been regretting it ever since that day.
It is also revealed that due to Genbu's death, Japan lost to Britannia because they had no leader. It is something that haunts Suzaku to this very day, and it causes him to be in conflict with himself, wanting to save lives, yet kill others at the same time.