|That's not hinna at all!
This article is a stub. You can help Golden Kamuy Wikia by .
Like his brother, he has a well accented face with a thin, pointed nose. His eyes have the rounded shape of black color equal to her hair cut very short. He is seen wearing the uniform of his ward.
In Meiji 38 (1905), Nikaidou took part in the Battle of Mukden during the Russo-Japanese War. When 2nd Lieutenant Tsurumi and Hajime Tsukishima were severely wounded, Nikaidou, along with several 7th Division members carried them to a frozen body of water to be carried away on sledges for treatment. They came upon Sugimoto with a dead body who offered to let them use the sledges with one of the Nikaidou twins asking if he was sure. They then brought the two wounded soldiers away to have their wounds treated.
During a blizzard storm, the 7th Division took shelter in a wooden building. While Genjirou Tanigaki and Tsurumi were having a somber conversation, one of the brothers whispered something into the other brother's ear.
Kouhei and Youhei first appear when they attempt to catch Sugimoto in a soba restaurant. Sugimoto immediately attacks the twins, lunging at them and knocking off their feet. Nikaidou twins order Sugimoto to get on his knees so they can shoot him. Kouhei whacks Sugimoto on the back of his knees with the butt of the rifle, making him fall on the ground. He hits Sugimoto repeatedly with a rifle, but Youhei tells him to step back so he can kill Sugimoto. However, the twins are stopped from killing Sugimoto, as First Lieutenant Tsurumi intervenes.
After Sugimoto is questioned and tortured by Tsurumi, Youhei and his brother pay a visit to Sugimoto in his cell. They ask if Sugimoto's still alive, and Kouhei remarks they might have got the wrong guy since Sugimoto got caught so easily. Youhei looks at Sugimoto's face and says that despite taking a real beating, Sugimoto's face is already healing. He remembers the rumors of Sugimoto the Immortal recovering overnight from injuries the doctors thought were fatal. Youhei wonders if cutting out Sugimoto's guts would result in it just being all healed by tomorrow. Kouhei advises his brother against it, saying that if they kill him they won't be able to find out where he hid the skins. Youhei takes out his bayonet and suggests they cut off Sugimoto's fingers one by one until he reveals the skins' location. Sugimoto whispers something, and Youhei gets closer to him to hear what he says. Sugimoto promises that he'll give him a mark so he can tell the twins apart and headbutts Youhei, knocking his front tooth out and making him fall onto Kouhei. Sugimoto takes a deep breath and lunges himself into the twins, breaking the chair and taking Kouhei into a chokehold with his legs. As Youhei stabs him in the chest, Sugimoto continues to fight, once again declaring himself Sugimoto the Immortal.
The fight between Sugimoto and the Nikaidou twins is stopped by the soldiers who heard the ruckus. Lt. Tsurumi orders to keep the twins away from Sugimoto. However, the twins visit Sugimoto once more to take their revenge. Youhei tells his brother to wait outside and make sure nobody comes in while he kills Sugimoto and later makes it look like Sugimoto was attempting to escape. Kouhei advises his brother to not use the gun and instead use his bayonet. When Kouhei leaves with Youhei's gun and shuts the door, Youhei finds his bayonet missing. He turns around and sees Sugimoto charging at him while holding a knife. Some moments later, Tsurumi arrives at the scene and witnesses the aftermath of the fight: Kouhei is crying, Youhei is dead and Sugimoto is heavily injured, with his intestines sticking out of his gut.
While Tsurumi is waiting for his horse being prepared, he investigates Youhei's death site, which he finds strange. He notices a wound on Youhei's right hand and wonders if he really stabbed Sugimoto with it or Sugimoto just put it in his hand. Tsurumi sees bloody fingerprint on the underside of Youhei's belt. As a drop of liquid runs down Tsurumi's face, he wonders why Sugimoto readjusted Youhei's belt after killing him. In the meantime, Sugimoto gets up on the sledge and pushes down the soldiers that were transporting him. Tsurumi unbuttons Youhei's jacket and realizes that Sugimoto took the dead man's intestines and tricked them.
- In 2018 Golden Kamuy All-Stars popularity poll Youhei ranked 43rd, gaining 797 votes. 
|7th Division Members||Hajime Tsukishima • Kazuo Maeyama † • Kennosuke Mishima † • Kouhei Nikaidou • Mokutarou Kikuta † • Otonoshin Koito • Tokishige Usami † • Tokushirou Tsurumi • Youhei Nikaidou †|
|Deserters||Fumio Okada † • Genjirou Tanigaki • Houzou Tamai † • Hyakunosuke Ogata • Ikutarou Komiya * • Naoaki Noma † • Rikimatsu Ariko|
|Allies||Heiji Koito • Hidenobu Okuda • Narizou Arisaka • Teruchika Yodogawa • Yasaku Edogai †|
|Associates||Kouji Wada † • Koujirou Hanazawa † • Yuusaku Hanazawa †|