Suga was born in Hiroshima, the first son in his family. Although the family held Buddhist beliefs, his younger brother Giichi converted to Christianity and became a Protestant missionary: he worked at churches in Manchuria, Vancouver Island, and Chicago and became the headmaster of YMCA in Tokyo. As a youngster, Suga attended the services at the Alliance Church of Hiroshima which was started by a Danish priest. Hudson Southwell, an Australian missionary interned in Borneo, later wrote: "During our time in the internment camp, Colonel Suga had often come into the church services in the women's section and sat near the back. Once he told Winsome wife directly, 'I'm a Christian.' This was a startling admission for a Japanese officer to make to a prisoner during wartime."