Saichi Sugimoto is a former soldier who fought in the Russo-Japanese war with the 1st division of the Imperial Japanese Army and got the name 'Immortal Sugimoto' for his invincibility on the battlefield. He lost his best friend in war who requested him to take care of his family. After returning home once his enlistment term in the army was over, he set out to pan gold in Hokkaido in order to earn enough money to fulfill his friend's last wish. There he met a man who told him a tale of lost gold treasure which once belonged to a tribe known as 'Ainu'. He also told him that the man who hid it was later imprisoned and he left clues to find the treasure on the skin of his fellow inmates.

Sugimoto is a strong and skilled warrior and soldier with instincts needed to survive. He even succeeds to overpower and kill a rogue beer with his knife and bayonet. Although fierce on battlefield and having blood on his hands, he is not cruel and unkind. He is valorous and kind. He later reveals on being asked why he is called 'Immortal Sugimoto' that he learned the only way to live is, not to be killed and so he only kills to protect his own life.

He has two scars on his face which are perpendicular and runs across his whole visage, while the third deviates from the horizontal one like a tree branch. His whole body is also covered in scars earned in battle.

Year round, Sugimoto wears a kimono with the sleeves tied up by a tasuki and the back hem tucked in shirihashori style over a mandarin collar shirt. He also wears the military cap, trousers, and high boots that were part of his uniform during the Russo-Japanese War. In colder weather, he also wears a dark blue trench coat and a foreign made scarf.

(Source: Golden Kamuy Wiki)