This manga tells the story of Giovanni Rorutesu (real surname was probably Roltes), an italian knight of the XVI century who travelled to Japan and served under the daimyō Gamou Ujisato, He later changed his name, and became Yamashina Rokurozaemon Katsunari. He fought in various battles, first as a soldier and then as a commander of many troops.
He came briefly back to Italy in 1584 as a messenger, to buy gunpowder weapons, and he also met the Pope.
He had blue eyes and red hair, some physical traits never seen amongs the japanese, but also not very common in Italy or Europe either.
Some people said he was a brilliant military strategist, probably even better than Zhuge Liang.
He's said to have died on the Vietnam coast, during a retreat after a battle.