|“||How dare you ask why! You betrayed ME! We were like brothers but you took everything and left me with nothing! NOTHING! Now I take everything from you including your life.||„|
|~ Yukio Mashimi to Mortu and Hamato about his betrayal.|
Yukio Mashimi was Yoshi's best friend, adoptive brother, and childhood friend. He and Yoshi consider themselves as brothers sometimes and they were both raised by the Ancient One. Mashimi was a guardian of the utroms before he became a Foot ninja when he joined the Foot Clan.
In Japan during the 1960s, Yoshi and Mashimi were orphans on the street begging for food and money. One day, they found a five yen coin that fell from a short old man named the Ancient One. Mashimi told Yoshi that they should keep it but Yoshi refused and returned it to the Ancient One.
The Ancient One was surprised of Yoshi's honesty that he offered Yoshi and Mashi that they can keep the coin if they can snatch it from his hand. Mashimi failed to snatch the coin but Yoshi succeeded. The Ancient One offered him to buy food but Yoshi requests that Mashimi can join too. The Ancient One reluctantly agrees and takes them to his home and gives them food. He raised them and taught them ninjustu.
Yoshi and Mashimi fell in love with the same girl named Tang Shen who also was raised by the Ancient One. Tang Shen only loved one of them, Yoshi, and Mashimi was angered by this. One night while the three of them were out exploring Tokyo, they came across two foot ninja attacking Mortu, in his human disguise, then Yoshi and Mashimi try to defend him, but his mechanical gets cut off. After the fight Mortu offers them both a chance to become Guardians for the Utroms, which, the Ancient One was against it, they accepted. However, while Yoshi rose through the ranks of the guardians, Mashimi failed to be promoted. Eventually Mashimi's jealously turned into resentment, until he snapped and murdered Tang Shen. Mashimi then joined the Foot Clan and helped lead them to the Utrom's hiding place for an attack, but the Utroms and Guardians were able to elude them with their escape pods. The Shredder then became furious at Mashimi for not telling them about the escape pods, even Mashimi tells him he didn't know about them.
When Yoshi learned that Mashimi murdered Tang Shen, he became enraged and made his way into Foot headquarters to challenge Mashimi. Mashimi at first thinks that he will not have to face Yoshi alone, but Shredder tells him that he will face Yoshi alone to redeem himself for Utrom's escape. Mashimi then duels Yoshi until he is fatally wounded by Yoshi and dies but not before asking Yoshi for forgiveness.