Splinter was the pet rat of Hamato Yoshi, one of best warriors in Japan's Foot Clan. Yoshi kept Splinter in a cage in and it amused Yoshi to watch his pet rat observe and try and mimic his movements as he practiced Ninjitsu.
When Yoshi killed a member his clan in a feud over the love of the lady Tang Shen, Yoshi took Shen and his pet rat to begin a new life in America, opening a small martial arts school in New York. Years later Oroku Saki, the brother of the man he had killed came to the city to found a western branch of the Foot and in his new guise of the Shredder tracked down and killed both Yoshi and Shen in cold blood.
In the fight Splinter's cage was damaged and he was left alone. Wandering the streets he saw a road accident involving a truck carrying toxic waste. Out of the truck came a canister which struck and shattered a young boy's glass bowl carrying four baby turtles. The rat followed the canister and the turtles down into an open manhole and observed how the turtles were crawling through the substance oozing out of the now broken canister. Not knowing exactly why he gathered them up getting ooze on himself.
He awoke the next morning to the see that both he and the turtles had doubled in size, all of them were also growing intellect soon learning to walk upright. The turtles followed him everywhere except to the surface but one day he received a shock, one of the turtles spoke the word "Splinter" and in doing so named him. In turn, out of battered renaissance art book he found in a storm drain gave them the names Leonardo, Michelangelo, Donatello and Raphael. He decided to train the turtles in the art of ninjitsu with the intention of one day charging them with the mission to avenge his master's death. When the turtles were thirteen years of age Splinter told his sons of his master Yoshi and of their true origin. Now that he was old and believed not long for the world he asked that the turtles track down and kill the Shredder.
Not long after the death of the Shredder Splinter became concerned when watching a news broadcast about a scientist named Baxter Stockman who promised to rid the city of it's rat problem using his robotic Mousers. These fears were proved to justified as while the turtles were off with their new found human friend April O'Neil confronting Stockman the Mousers raided the lair.
Badly wounded in a fight with the robots and fleeing he was found by two workmen working for an organization known as Techno Cosmic Research Institute (TCRI) and they took the rat back to their building. Waking up in a bed he wandered the adjoining corridors and was shocked by the site of TCRI workers as brain-like aliens inside the stomach cavity of robotic human bodies. Thankfully these aliens called the Utroms were peaceful, not only that but they were the ones involved in the road accidents which caused the canister of toxic waste by their technology to mutant him and his sons.
Exile in Northampton
After being reunited with his sons he briefly moved to April O'Neil's apartment with them. On Christmas Eve however the apartment was raided and burned down by the resurrected Shredder and the Foot Clan. Moving to a farmhouse in Northampton which used to belong to Casey Jones's grandmother. Splinter noticed their loss at the hands of their enemies weighed heavily on April and his sons and so as Spring set in he took the turtles on intense training in the woods of Northampton to purge them of the inner demons they'd picked up.
Shades of Grey & City at War
Splinter was not devoid of demons of his own, after years on the farm Splinter would be distant and blunt with those around him. On one occasion April poured her heart out to Splinter about her feelings of isolation, asking his advice he coarsely answered "leave" before slamming the door behind him. After the turtles, April and Casey had left the farm Splinter was left alone but this did not temper him. In the woods he finds himself continually unable to focus his thoughts and meditate, out of frustration he lashed out at his surroundings.
Splinter ventures to an old abandoned factory. Climbing to the top a silo the bricks give way and falls inside and awakes with a broken leg. Splinting his leg but unable to stand he worries that with winter approaching and no food he can't afford the time for his leg to heal and escape. Some time later after erecting a crude shelter he is awaken in pain to a voice which taunting him. Begging for help, the voice tells him that the means for his survival is within is reach but not yet within his grasp. Splinter becomes convinced that the voice is death, the voice answers that he is not death but he knows death, stepping out of the shadows he reveals himself to be the Rat King.
Over the period of a couple days to a week, the Rat King belittles Splinter. Eventually, he convinces Splinter to turn on his brethren out of desperation for food; Splinter grabs a Rat, breaks it's neck, and eats it. The Rat King stresses that sometimes one's beliefs blind yourself from seeing all possibilities. The Rat King also states that they will meet again. After recovering from his broken leg and his deranged state, Splinter discovers the corpse of the Rat King in tatters and obviously there for a long time. Splinter manages to muster enough strength to climb out of the silo.
Post City at War and Onwards
After the fallout from the battle with the Elite Foot, the turtles venture to Northampton to meet up with Casey, April, and Shadow. At this point, Donatello admits that he will stay behind to go on a mental pilgrimage with Master Splinter. Before anyone can say their goodbyes to him, Master Splinter disappears. He later shows himself to April and says "Daughter" before hugging her and returning to the woods.
Splinter remains in Northampton and never returns to New York City. His days are spent at the farmhouse often enjoying the quiet moments in life and meditating. At one point, he discovers Shadow's boyfriend spying on the house and asks Mr. Braunze to erase the boy's memories.
While making tea, Splinter has a heart attack. A flash of the Rat King silently appears before Splinter. Splinter crawls into his room and falls to the floor of the closet. With his last breath he whispers "...sons..."
1987 TV Series
In this series, Master Splinter, instead of portraying the Mirage Studios Idea of Splinter being a pet of Hamato Yoshi, Splinter was Hamato Yoshi. Before meeting the turtles, back when Yoshi lived in Japan, there was a Ninja Clan known as The Foot, which was led by Hamato Yoshi. One of his students, Oroku Saki constantly challenged him in an effort to usurp his leadership, finally causing him to be banished from the Clan. Yoshi fled to the United States in poverty. He was forced to live in the sewers, where he befriended the rats. One day, a small child was returning from a pet store, carrying four baby turtles in a fish bowl, when he tripped and fell, causing the bowl to crash on the street and the turtles fall into a drain. Yoshi had found four new friends.
One day, Yoshi discovers the turtles are covered in "goo", which he later realizes was actually some kind of mutagen that caused whoever touched it to take the form of the animal it most recently came in contact with. The turtles became humanized, with their recent contact with Yoshi (with his attempt to clean them up), and Yoshi becomes a mutant rat with his recent contact with rats. Since then, Yoshi raised the Turtles by himself, and gave them the names of his favorite Renaissance artists: Leonardo, Donatello, Raphael and Michaelangelo, who were to be distinguished by personality and headband color. He also taught them the art of Ninjitsu in order to protect themselves better, since people above ground would not accept them.
Hamato was given the nickname "Splinter" by his students, due to his proficiency at breaking wooden boards. On one occasion Donatello was able to use the remnants of the Ooze that transformed them to restore Yoshi's human self, but the process was only temporary and soon wore off; however, Splinter, having seen how humans could treat each other at times, concluded that he preferred his new state, particularly due to the relationship he had with his students. He was voiced by Peter Renaday.
Splinter represents the emotional stronghold for the Ninja Turtle family. He is a strict father to the turtles but loves them dearly. He emphasizes meditation, deep thought, honor, and the virtues of Bushido to his sons.
Splinter is often depicted with a wooden cane, but it is used for more than just an aide to walking. He is a master of ninjutsu and skilled in multiple areas of weaponry. However, his primary abilities lie in psychic activity. Often, Splinter has waged mental battles and struggles with Foot Mystics or some other similar threat. These often take a physical toll on Splinter, but it deepens his knowledge in the mystic arts. He is such a master of the arts, he has gained the ability to float while meditating.