Hamato Yoshi

Hamato Yoshi is a ninja who is always close to Splinter. He is usually dead when the story starts.

Eastman and Laird's TMNT
In the comics, Hamato Yoshi is a highly-ranked member of the Foot Clan in Japan. Splinter is his pet, who copies him as he practices Ninjutsu. When he got romantically involved with Tang Shen, he invoked the jealousy of Oroku Nagi, who also loved Shen. When Nagi got enraged with jealousy, Yoshi was forced to kill him to protect Shen. Dishonored for having killed another member of the clan, Yoshi was forced to choose between exile and death. He choose exile and moved to New York City with Shen. Years later, Oroku Nagi's brother, Oroku Saki killed them both to avenge Nagi. Splinter escaped to the sewers, where he and the Turtles were mutated. He taught them Ninjutsu in order to avenge the death of Hamato Yoshi.

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 here family and she is not a member of the Oroku family.

1987 Series and Archie Comics
Like in the earlier comics, Hamato Yoshi was a member of the Foot Clan. However, Oroku Saki frames Hamato Yoshi for the death of one of the Foot's Clan leaders. Yoshi is exiled to New York City, where he is forced to live in the sewers and befriend the rats. Years later, when Saki moved to New York himself, he dumps the mutagen into the sewers to kill Yoshi. However, the mutagen mutates Yoshi and his four pet turtles. Yoshi turns into a giant rat, the last animal he was in contact with, and takes on the name Splinter, "for obvious reasons".

Movies
The movies follow the story of the original comic books, except the conflict is with Oroku Saki himself rather than Oroku Nagi. Instead of fleeing the clan because of a murder, Tang Shen convinces Yoshi to move to America in order to avoid dueling Saki, probably to the death. Saki vows vengeance and eventually kills both Yoshi and Shen, leaving Splinter homeless.

2003 Series
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 of 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 escaped. Soon after, he discovered Tang Shen dead. In anger, 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.

It's revealed in the lost season that Yoshi and the Utrom tried to get the Ninja Tribunal's help in the fight against the Utrom Shredder, warning them that the alien may challenge the evil one. Due to their refusal to help, Splinter partially blames them for Yoshi's death.

When the Tengu Shredder was on the verge of defeat, Yoshi manifests himself through the Turtles' medallions, which shatters them and then slayed the demon in half.

Yoshi was also a champion of the Battle Nexus.