In most versions he was the owner of a pet rat who became Splinter; this includes the original comics and Teenage Mutant Ninja Turtles 2003 cartoon series. The earlier 1987 cartoon series (along with the Teenage Mutant Ninja Turtles Adventures comics) provided a twist in that Hamato Yoshi was Splinter, having been turned into a giant rat. This revelation is to also set in the 2012 TV series.
Eastman and Laird's TMNT
In Tales of TMNT #22, the Turtles encounter a female ninja with a Shredder gauntlet. She vows to kill the Turtles for crimes Hamato Yoshi committed against her family.
1987 Series and Archie Comics
The Archie comics gave more backstory to Hamato Yoshi when he, the Turtles and April visited Japan. He recounted his youth training under his Uncle Giyogi near the outskirts of Hiroshima, and personally saw the devastation of the city after the atomic bomb was deployed.
The incarnation in the 1987 animated series was depicted as more of a teacher than a father figure to the Turtles because of the Turtles being mutated to fully grown forms in this continuity. The Archie Comics, however, made it so that the mutagen only mutated the turtles and they grew up like normal people as in all other continuities.
Yoshi's story, explained in the first movie mirrors the story of the original comic books, except Oroku Nagi is removed from the story and the conflict is with Oroku Saki himself rather than Oroku Nagi. Instead of fleeing the clan because of a murder of a clan member, Tang Shen persuades Yoshi to flee to America in order to avoid dueling Saki, (presumably to the death) for her hand in marriage. Saki vows vengeance and spent years searching for them. Saki eventually finds and kills both Yoshi and Shen in their New York apartment, leaving Yoshi's pet rat Splinter homeless. The actor who portrayed him was uncredited.
In the third Teenage Mutant Ninja Turtles film, the turtles save a child named Yoshi from a burning building in 17th century Japan, Michaelangelo getting him out of the building and Leonardo subsequently performing CPR. This Yoshi's relationship with Hamato Yoshi is never fully explained, but based on the past incarnations of another character that the turtles meet on this adventure (such as a possible British ancestor of Casey Jones, Whit) a direct relationship between this boy and Hamato Yoshi seems implied. If this is indeed the case, then when the turtles save the boy's life, they are also saving their own future, as their origin is inexorably tied to the fate of Hamato Yoshi.
Hamato Yoshi's story has been fully revised in this version. The story is revealed in A Tale of Master Yoshi.
Hamato Yoshi was an orphan, living in the streets of Japan with his best friend, Yukio Mashimi, in the 1960s. When Yoshi returned a 5-yen coin dropped by the Ancient One, he took them in for their honesty, raised them as his sons, and taught them Ninjutsu. They both fell in love with Tang Shen, another orphan taken in by the Ancient One. Shen loved Yoshi more. During this time, Shen found a rat in the house and kept him as a pet.
One night, Yoshi, Shen, and Mashimi saw a lone man being attacked by four ninjas. Yoshi and Mashimi decided to help the man and fought off the ninjas. The man was actually the Utrom Mortu and the ninjas were members of the Foot Clan. Impressed by their skill and already having his secret revealed due to battle damage, Martu offered to make the two Guardians. They accepted, despite their master's objections.
As a Guardian, Yoshi seemed to advance continually, while Mashimi never seemed to get anywhere. In anger, he murdered Tang Shen and betrayed the Utroms to The Shredder. Yoshi and the Utroms managed to escape with their lives. When Yoshi returned to the Ancient One, he found his master cradling Tang Shen's body. In anger, despite counsel from his master, Yoshi stormed the Foot's citadel and killed Mashimi in single combat.
With their Tokyo base discovered, the Utrom set up a new base in New York under the name T.C.R.I.. Yoshi went with them, taking Shen's rat with him. He named the rat Splinter to remind him of the splinters of anger and hate that drove him to kill Mashimi.
However, the Shredder followed Yoshi and tried to force him to reveal the location of the Utroms. Yoshi resisted and was killed. Splinter escaped to the sewers, where he mutated along with the Turtles. He proceed to teach them Ninjutsu, just as he had learned from Yoshi.
Yoshi was also a champion of the Battle Nexus.
It's revealed in The Lost Episodes that Yoshi and the Utroms tried to get the Ninja Tribunal's help in the fight against the Utrom Shredder, warning them that the alien may channel the evil one. Due to their refusal to help, Splinter partially blames them for Yoshi's death.
In episode, "Fathers & Sons", Splinter had procured Yoshi's ashes after his passing and made great strides to bring himself and the then-young Turtles to Japan. Yoshi's ashes were then laid to rest next to Tang Shen.
When the Tengu Shredder goes to attack Splinter, Hamato Yoshi manifested from the Turtles' medallions. He snatches the Shredder's trident and then slashes him with it, causing precious energy to leak from his armor.When the Tengu Shredder was on the verge of defeat, Yoshi manifests himself through the Turtles' medallions again, which shatters them and then he slayed the demon by slicing him in half.
When you beat the very first video game Splinter turns back into Yoshi. Yoshi is an unlockable playable character in the 2003 video game. In the TMNT GBA game you get a Hamato Yoshi Statue Trophy, for finishing all acts and obtaining a total high score.
Hamato Yoshi is a member of the Foot Clan in feudal Japan. When fellow clansman Oroku Saki advocated the murder of innocents, Yoshi left the clan. In retribution, Saki had his wife, Tang Shen, murdered. With her dying breaths, Shen told Yoshi flee with their four sons. Years later, Saki and his ninja caught up to Yoshi and his sons, killing all of them. Before he was slain, he prayed to Buddha and all other powers of the universe that he and his sons would be able to come back for revenge on Saki. In 2011, they were reincarnated as Splinter and the four Turtles.
2012 TV Series
Splinter took the now fully grown up turtles into the sewers and decided to live a life in hiding with them. He raised them as if he were their surrogate father, and he trained them in the ways of ninjutsu. Although Splinter has adjusted to his new life and identity, he still holds his human memories dear. He's often seen looking at the picture of his wife and his long lost daughter, Miwa (who is actually soon revealed to be Karai: Oroku Saki's adoptive "daughter").
- Hamato Yoshi's given, "Yoshi", (喜), translates as "like, love, enjoy", or "joyful thing". Hamato Yoshi's surname, "Hamato", (濱戸), translates as "beach/sea coast/river bank door/family". If written purely in hiragana, "Hamato Yoshi" means "Alright still".
- Though Hamato Yoshi never had children, in second volume Tales of TMNT issue 34, it was revealed that he did have a niece Hana. But after revealing her true origins she's never heard from again.