Yatsume Mumyōi is a member of an old gold mining clan. Family tradition led them to elongate the limbs of the family members using iron rings to help them mine better.
Yatsume took up fighting and joined the Yaminobu Shadow Warriors that tried to kill Kenshin years ago. He joined Enishi in order to support his clan, who had fallen under hard times.
When Kenshin sees and defeats him, he vows revenge and returns in the Jinchū arc under Enishi to extract vengeance.
However, before he can fight Kenshin, he ends up fighting Saitō instead and is easily defeated. After Yatsume is defeated by Saitō, Kenshin offers to let Yatsume fight him if he still wants to but suggests that he go up north and use his abilities to help his clan. He is arrested by the police afterwards.
Of all those involved in the Jinchū, he appears to show the most respect to his fellow comrades, formally greeting them (albeit from a hole in the ceiling). He also accepts Kenshin's generosity to go on with his life. He is also one of only three of the "6 Comrades" who was actually obsessed with taking revenge on Kenshin.
Yatsume's appearance is very similar to the super villain Venom the American superhero the Spider-Man villain. Nobuhiro Watsuki admits in Yatsume's character notes that his design was partially based on Venom.
Yatsume's limbs are about 1.5 times as long as normal, with the notable exception of his left arm, which is twice as long and equipped with a metal claw. His teeth have also been sharpened into fangs from a young age.