Sugimoto will die of cancer.Specific mention has been made of him recovering of wounds almost overnight. This abnormally high healing rate may be a sign of him having a slightly faster than normal cell cycle, which indeed makes for faster healing of wounds, but also makes for an increased risk of developing tumors. Given exactly what his healing rate seems to be, he may not get to die of old age even if neither man nor beast manages to kill him. His utter lack of fear of death since he entered the Army —which Umeko actually noticed as "the smell of a different man"— could also be caused by a small brain tumor in its initial stages of development causing a change in behavior.
The above speculation was true, but Sugimoto will no longer die of brain cancer.Ogata's bullet took away the tumor.
Kōichi Hasegawa is Lieutenant Tsurumi.As Golden Kamuy Hunting finds out, there are some facial similarities between Hasegawa and young Tsurumi that become clear when one removes the former's eyeglasses and the latter's facial hair. Hasegawa may not be some minor character because he looks too ordinary in a Cast of Snowflakes. Being a photographer and marrying a native is not bad for a spy either. Furthermore, Hasegawa being Tsurumi would also explain why Tsurumi knows young Kiroranke helped assassinate Tsar Alexander II.
- Confirmed in Chapter 179. When his wife, Fina, was dying of a stray bullet, Hasegawa told her the truth that his real name was Tokushirou Tsurumi.