(So my friend pointed out that I hadn't posted this here yet..)
Spinter was the pet rat of Hamato Yoshi. Blessed with higher than usual intelligence and dexterity (it may be said that he was aready a mutant), he learned the way of the ninja from watching his master.
When Yoshi fled to New York, hiding in the hold of a freighter, Splinter came with him. When Yoshi was attacked by The Foot and Oroku Saki, Splinter leapt to the defence of his master, scratching Saki's face. Splinter received a slice to the head which took a notch out of his right ear and was left for dead.
Escaping the burning building minutes later by the sewer, he came upon four baby turtles crawling in a glowing green ooze. Later, he named them from an article in the New York Times on an art exhibit.
A little known fact is that Splinter was originally called 'Nezumi', but changed his name following the death of his master, vowing to be "a Splinter in The Hand" until the clan was restored to its true ways.
Splinter is a master of ninjutsu and is versed in is most mysticof secrets. As a follower of Mikkyo he can see the pattern of destiny laid out before him and make decisions to conciously affect that pattern. This is the true power at the heart of the way of the ninja.
Design-wise he was a tricky one. I feel he should always be in perpetual shadow, just a voice in the darkness. A click of his sword now and again, a glint of an eye. Having said that, a black page wouldn't really give any design cues. He is dressed traditionally in gi and hooded cloak. He looks unarmed but carries a shirasaya sword cane and is loaded with dozens of concealed shuriken, smoke bombs, etc.
Splinter knows that it is for his sons to take down The Shredder and restore balance to The Foot, but he moves out of the shadows to face his enemy, knowing that his death will motivate his sons to move to a higher level of understanding.
A ninja stands on the boundary between this world and the next. The power of life and death lie equal in his hands.
