Some TMNT stuff was always meant for grownups.
In the original comics, Splinter was the pet rat of a ninja named Hamato Yoshi in Japan. Intelligent for his species, in his cage Splinter was able to learn his master's art by mimicking his movements while he practiced. Yoshi became embroiled in a dispute with a fellow ninja by the name of Oroku Nagi. When Yoshi had to go to America into exile, years later he was eventually murdered at the hands of Nagi's brother Oroku Saki, who sought revenge for the death of his brother Nagi.
Without a home, Splinter was forced to run away and live in the New York City sewers. Due to a traffic accident, four baby turtles and a canister of radioactive ooze were sent down into the sewer, where they were exposed to the strange substance that mutated both Splinter and the turtles. Within hours Splinter and the turtles were intelligent, humanoid mutants. Splinter named the four turtles: Leonardo, Donatello, Raphael, and Michaelangelo (after a book on Renaissance artists that he found in a storm drain) and in the following years trained them in the arts of Ninjutsu so that they should take revenge for his dead master, while still raising them as his own sons. Although Splinter at the beginning was very careful to keep his existence and his foster sons secret from the rest of the world rates, the four little turtles made friendly contacts with the upper world, in particular with April O'Neil and Casey Jones.
Splinter was usually more in the background instead of at the forefront. He stood aside the Turtles always with advice, but when it came time for a direct fight, he showed despite his age a high skill as a ninja master. In volume 4 issue #10, it would have appeared that this version of Splinter had died of a heart attack. For all his friends, it was a big shock when Splinter unexpectedly died; much later it turned out that he had found a new home in the Battle Nexus. How he got there however, remains unclear. However, as revealed in Teenage Mutant Ninja Turtles issue 32 (volume 4), the Splinter that died was not the Splinter that raised and trained the Turtles.
In V4 #26, Splinter's full formal name is revealed to be Hamato Splinter, taking the same Japanese surname as his master Hamato Yoshi.
Splinter is a stern, yet patient, sensei. He had devoted his entire life to a single goal; gaining vengeance for his beloved master Yoshi. Once his sons had achieved this goal, Splinter found himself somewhat bereft of purpose. Therefore, he spent much of his time in meditation, contemplating his place in the larger scheme of life.
Splinter will often use his skills of stealth and deception to avoid confrontations and trouble. However, when forced to intervene directly in a situation Splinter acts with focused and precise action. He wastes little motion and attempts to defeat his foes with as little effort as possible.
At heart though, Splinter isn’t the killer that he raised his sons to be. Outside of members of the Foot Clan, Splinter will avoid taking a human life if at all possible.
Though Splinter is the head of his family, he allows his sons to take the lead and make decisions on their own. He rarely interjects unless the brothers are unable to come to a consensus on their own (i.e., they are about to come to blows). Or if their actions have become self-destructive.
In those moments, he acts as both a wise sage and stern disciplinarian and brings their mercurial natures back in-line and focuses them on the task at hand.
- Splinter's longevity is unheard of even as a pet rat. Feral rats usually live less than one year, while healthy pet rats live an average of 1.8 years, with very few ever living longer than three years. One human year is equivalent to about 30 "rat years." Splinter was born unmutated in 1960, and did not mutate until he did so along with the turtles in 1970, which would have made him between 9 and 10 years old, or between 270 and 300 rat years, an impossibly long lifespan for any natural rat. He then went on to live for 32 more years and was still alive in 2002, the year his doppelgänger died of a heart attack. In any event, it was understood that Splinter was already elderly on his mutation day.
- Unlike other versions of the character, this version of Splinter has never confronted the Shredder himself, despite having a personal vendetta against him for killing his master