Kouhei Nikaidou is the Private First Class of the 7th Division and Youhei's older twin brother. 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 his hair cut very short. He is seen wearing the uniform of his ward.

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After the death of his twin brother Youhei, Nikaidou became focused on revenge against Sugimoto. He seems to have a strong craving for morphine as just the mere mention of it was able to get his full attention.

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Kouhei loses a multitude of his body parts throughout the story. He lost: His left ear to a bear when hunting for Tanigaki, his right ear when Tsurumi cut it off after catching him following said hunt, his right leg to Hijikata's ambush inside Edogai's house as well as his right hand when he tried to shoot Sugimoto after his failed ambush in Abashiri Prison. He gets prosthetic limbs for his leg and hand made by Narizou Arisaka. The prosthetic limb has a hidden gun in it.