Waichirou Sekiya is one of the Abashiri Convicts, a former veterinarian and an expert in poisons.

The fourth season of Golden Kamuy.
 
The race to obtain the lost Ainu gold is nearing its peak, as even more formidable ex-prisoners are set on keeping their tattooed skins intact. Saichi "The Immortal" Sugimoto and Asirpa can finally move forward together, but Asirpa's time with Kiroranke and Ogata Hyakunosuke has slightly altered her ambition and strengthened her determination. As their party leaves the Russian territory, they gain a peculiar follower.
 
Meanwhile, Toshizou Hijikata's crew makes major moves to get invaluable skins, falling into the poisonous trap of a dangerous convict. Although each group possesses a strong advantage over the others, even First Lieutenant Tokushirou Tsurumi—the most powerful and ruthless contender—cannot advance without Asirpa's help. However, his ominous plans for when they will inevitably meet remain a mystery.
 
(Source: MAL Rewrite)
 
Note: On November 7, 2022, it was announced that the seventh episode of the season and beyond would be postponed due to the passing of a main staff member. The season restarted broadcasting from the first episode on April 3, 2023, with the seventh episode released on May 15.

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In the early twentieth century, Russo-Japanese War veteran Saichi “Immortal” Sugimoto scratches out a meager existence during the postwar gold rush in the wilderness of Hokkaido. When he stumbles across a map to a fortune in hidden Ainu gold, he sets off on a treacherous quest to find it. But Sugimoto is not the only interested party, and everyone who knows about the gold will kill to possess it! Faced with the harsh conditions of the northern wilderness, ruthless criminals and rogue Japanese soldiers, Sugimoto will need all his skills and luck—and the help of an Ainu girl named Asirpa—to survive.
 
(Source: Viz Media)
 
Note: Won the 9th Manga Taisho Award in 2016 and the 22nd Tezuka Osamu Cultural Prize in 2018. Nominated for the 20th and 21st Tezuka Osamu Cultural Prize and the 40th Kodansha Manga Awards in 2016.