During the early-1960s, Japan was still recovering from its defeat in World War II and it was not uncommon to see orphans begging for food and money. One day, Hamato Yoshi and his friend Yukio Mashimi picked up a five yen that fell from a stout man. The latter was intent on spending it; the former returned it. Seeing such kindness, the old man, known as the Ancient One, told them they could have the coin if they could snatch it from his hand. Mashimi was unsuccessful; Yoshi was. The Ancient One took the two of them in, gave them food, trained them in the ways of ninjutsu and raised them as his own sons. They always did everything together and even fell in love with the same girl, Tang Shen.
While in the city walking Tang Shen, the two noticed a man being harassed by dishonorable ninjas (they soon learn it was the Foot Ninja) and helped him. But they discover his arm is mechanical. The man, Mortu, offered them the chance to join the Guardians or be left with the strange mystery; they chose the former but the Ancient One disapproved, saying they required mental training. But as men, they made the decision on their own and joined the Guardians. However, Mashimi failed to move up in the ranks as Yoshi became Guardian Prime. Soon, after it became clear that Tang Shen loved Yoshi, something in Mashimi snapped and he murdered her under the rain and then betrayed the Utroms' location to the Shredder. But he did not know about the escape pods and was on thin ice with the Shredder. When Yoshi returned to the Ancient One's home, he found Tang Shen dead. He knew Mashimi had done this and went to the Shredder's fort to challenge Mashimi, the latter was confident that he would be supported but the Shredder ordered him to fight Yoshi alone. Ultimately, the latter wins and Mashimi asks Yoshi to forgive him as he dies. Mashimi actually plays an important role as his actions and anger thereof helped in bringing the Ninja Turtles to existence. A hungry little rat who would come to be known as Hamato Splinter would have been disposed of on sight by Mashimi had Tang Shen not intervened. His jealousy and betrayal started the chain of events that led Yoshi and the Utroms to flee to New York.
- Yukio Mashimi was created for the 2003 series to replace Oroku Nagi, who could not have existed as a contemporary of Hamato Yoshi's, as the Oroku Saki of this time and series was an Utrom, rather than a human. Yukio's last name seems to be an anagram of Mishima. Yukio Mishima was a Japanese author who gained worldwide infamy when he committed seppuku in 1970.
- Yukio shares the same name "Mashimi" as Oyuki Mashimi's last name from the Archie Comics. Their first name are anagrams of each other.