Yatsume was member of a clan that made their living by mining gold and as a family tradition, they used iron rings on children to elongate their limbs to make them better miners. However the mine ran out of gold, leaving the clan on hard times, and Yatsume became an assassin, hoping to prove his battle skills to the Tokugawa government. He joined up with the Yaminobu Shadow Warriors to assassinate Himura Kenshin. When I’izuka was discovered as a traitor by one of the Chonshu party, Yatsume caught him and he was killed by Nakajo. As the Yaminobu began their plan to kill Kenshin, Yatsume attacked him while he was fighting Sumita, gouging into his shoulder. After Sumita got his legs cut off, Yatsume stabbed Kenshin in his other shoulder and asked if he should give the Battosai credit for persistence. However Kenshin stabbed Yatsume in the hand, saying he’d kill him soon, and he tried to pull the sword out, but couldn’t. The Battosai said it now made sense how Yatsume had such a reach and he pulled his hand up, slicing it partly in half in the process. He said his body was a secret passed down through the Yatsume clan and it was law that anyone who saw it had to die. Yatsume leapt away, saying he’d end their fight for now, but he’d kill Kenshin someday, be it in this era or the next. At some point, Yatsume joined Yukishiro Enishi’s campaign of revenge against Kenshin.
After Enishi began gathering his allies to begin his Jinchu campaign, Yatsume arrived at the mansion he’d rented in Yokohama without the others' knowledge. When everyone else had arrived, Enishi noted only one of them remained and Gein said he was told to come that night, but at no specific time. However Yatsume revealed he was already there, speaking to them through the ceiling from the room above them. Inui Banjin told him to come down and Gein said he should at least show his face, but Yatsume replied he didn’t wish to reveal his appearance. Enishi explained to them all that the Battosai now lived under the name “Kenshin” and killing him now would not be true revenge. He went on to say they had to corner him and make him recognize his crimes, then they could break ties to try and kill him. Enishi then explained the plan to them and Kujiranami Hyōgo, Otowa Hyōko, and Inui left to begin the first phase of Jinchu. Later on, when Enishi announced to his allies that they would be finishing Jinchu in ten days, he went inside the mansion and asked Yatsume if he’d heard that as well. However Yatsume asked Enishi what he was plotting and what Gein was working on in the basement, where there was no ceiling space for him to crawl in. Enishi replied he wasn't going to yell it up there and Yatsume could come down and meet him face to face. The assassin replied to Enishi that he didn’t care what he and Gein were planning and he had no intention of getting in their way. However Yatsume told Enishi not to get in his way either, because he was going to kill the Battosai, the only man who'd witnessed his appearance and lived.
Attacking Kamiya Dojo
As the ten days came to pass, Yatsume and the others were presented with upgraded weapons, before they left to start Jinchu. They arrived on hot air balloons and for awhile, Yatsume watched the others fight Kenshin and his allies. After Gein, Inui, Otowa, and Kujiranami were defeated, Yatsume went after Kaoru, but the former Battosai saved her. Yatsume noted that Kenshin’s eye color still changed when it came to women and the former Battosai said his arm was longer than before. The assassin explained that he’d spent 15 years lengthening his arm and said he was 16 when he'd started, so he had to be rougher since it was at the end of his growth spurt. Kenshin was surprised to hear that and Yatsume asked if he thought they were born like this. He explained the history of his clan and that it was law to kill anyone who saw the Yatsume body. Yatsume declared he’d kill Kenshin first and then everyone else there and told the former Battosai to pick up his sword, before asking if Kaoru would face him. Kenshin asked Yatsume why he entered a life of battle and the assassin explained that the gold mines started running out. To help his clan live, he’d realized the body shaping gave him great potential for battle and he’d wanted to prove this to the shogunate. But Yatsume had lost to Kenshin and thus his clan was on the verge of dying from starvation and exposure and were taking a chance by looking for gold veins in Hokkaido. Yatsume again demanded that Kenshin pick up his sword, when Saito showed up, telling the former Battosai to stand back. Yatsume asked the former Shinsengumi who he was, but Saito replied a human being realized when it came time to leave because of their intelligence. He went on to say someone who blamed his defeat on the victor, losing sight of his moment to leave, was lower that a beast and Yatsume was a freak, inferior to a beast. Angered at Saito’s words, the assassin told Kenshin he was going to borrow five minutes and snarled at the former Shinsengumi that he’d killed everyone who dared call him freak. Yatsume threw himself at Saito, crying no one would could call his clan freaks and struck, generating a fissure in the ground. The assassin declared that was he above human beings, but Saito asked if his fangs were part of body shaping. Yatsume said he had gradually sharpened them from a young age, cementing them with bone powder. Saito then asked about his long tongue and the assassin said it was his own, to which the former Shinsengumi said he was a freak through and through. Enraged, Yatsume said he’d kill Saito as he prepared to attack, but the former Shinsengumi took the stance for Gatotsu. The assassin said again that he’d kill Saito, but Saito just said that he was sick of hearing that and told him to hurry up and strike. Yatsume and Saito attacked each other with Yatsume slashing Saito’s side by sending up earth in his Earthen Fortress attack to slow the former Shinsengumi’s arm. The assassin said a gold-mining clan was always protected by earth and explained it was impossible for any strike to maintain its full strength when it came against the wall of earth and blow from his arm. Yatsume declared that with his elongated arm, the earth and shadows were in his grasp and it’d been 15 years since he went to chase the Battosai, now it was his moment of fulfillment. Yatsume told Saito he’d make him greatly regret rudely poking his nose into others business. However the former Shinsengumi said he still needed to have limits to his stupidity and called Yatsume a freak again. Saito said the assassin reacted to freak, but not fool, so maybe he’d become self-conscious about being one. Enraged, Yatsume again snarled that he’d kill the former Shinsengumi, but Saito replied he was tired of hearing that. The two attacked each other again and Yatsume asked Saito if he still planned to use that move and the result would be the same, no matter how many times he used it. Yatsume used his Earthen Fortress again, but Saito managed to catch the assassin’s claw with the tip of his blade, the one area that was not protected by the Earthen Fortress. Yatsume said the former Shinsengumi was quite resourceful to identify that target, but unfortunately his left arm moved faster and more precisely than a machine. However Saito said his plan had just begun and thrust his sword into Yatsume’s arm with Gatotsu Zero Shiki. The assassin screamed in pain at his mangled arm and the former Shinsengumi told him to stop crying so loud and be thankful that his shoulder didn’t fly off. However Yatsume pushed the sword into his arm more, turning it into a splint and Saito asked him if he planned to continue. The assassin said he’d lived the past 15 years to kill the Battosai and he’d rather die than leave without fighting him. Saito just told him to hurry up and die, but Yatsume leapt away under the dojo to a cache of landmines, which Enishi gave him. Yatsume leapt out to face Saito again and began burrowing into the ground, forming a field of containment with the landmines. Kenshin realized what it was and the assassin complimented his intuition, explaining that his landmines exploded when stepped on or activated. Yatsume activated some of them, then asked Saito how the fear of not knowing where they might explode, the fear of not being able to move a step, and the fear of being attacked from underground was. Yatsume told Kenshin to watch carefully and this was one more person who got involved in his life would die. The assassin told Saito this was his end and set of all the bombs at once, leaping out of the ground at the same time. However the former Shinsengumi got behind Yatsume, grabbing his head and smashing him into the ground. Saito pulled his sword out of the assassin’s arm, much to his pain and said someone who lost sight of the big picture by living in a personal life didn’t qualify for death. Saito went on to say that if Yatsume would rather be dead, he’d grant his wish, but Kenshin stopped him from killing the assassin. The former Battosai said he wouldn’t let anyone in his sight die and told Yatsume if he still wanted to fight him, heal and come back since this was originally a personal affair. However Kenshin told Yatsume to travel north to help his clan, since his skills in the earth would be a great help to them, then asked Megumi to treat the assassin’s wounds. Afterwards, Yatsume was arrested by the police.
Because of his clan’s tradition, Yatsume’s limbs are one and a half times as long as normal, except for his left arm, which is twice as long. It allows him to strike out from distances people normally couldn’t and he wears claws on his hand to kill his targets with. Yatsume is able to use the earth around him as a weapon, namely through his Earthen Fortress technique, where he grabs up a handful of earth, then throws it up to take away the momentum of his opponent’s attack. Yatsume can also burrow through the ground with seeming ease and is quite skilled at stealth, demonstrating the ability to crawl on surfaces.
- Yatsume looks extremely similar to Venom, especially before his appearance was fully revealed. Nobuhiro Watsuki, creator of Rurouni Kenshin has noted his design was partially based on Venom.
- Yatsume’s last name, “Mumyōi” actually means “nameless”, which means he has no given name.