Leonardo began his life in a pet shop in Manhattan, a simple inoffensive turtle seeking no more in life than the next leaf of lettuce. It was a simple life, but a very good one. The store was warm, there was ample lettuce, and nobody really bothered him or his clutch brothers much. This all changed when a human adopted him and his brothers and carried them away from the shop, only to run afoul of a catastrophic accident. It's impossible to describe what Leo thought while the cage was falling into the sewers. Some disquiet, a bit of apprehension perhaps at the sudden drop, but he had his shell, his brothers and there was still lettuce, so all was okay.
It's a terrible thing to suddenly become self aware. First all you want is a bit of lettuce, next you start asking questions like where does lettuce come from? What is lettuce, really? And who, when you get right down to it, is doing all this thinking? From quadruped bliss to abstract conceptualizing… talk about a change in perspective.
Over the course of the years, Leo was able to piece everything together, aided by Master Splinter, the mutated human who had adopted him and his brothers. Not that he was still a normal turtle himself, having mutated in the same accident that created Splinter. What had happened topside nobody really knew, let alone what the ooze that had transformed them all had been, but they did know that is was probably best not to be seen any time soon. Soon enough Leonardo and his brothers were learning Ninjutsu from Master Splinter, and taking to fighting the criminal gangs of the city. Why? Because someone has to. And when a ninja clan starts to wreak havoc and nobody else can deal, who else but a ninja to put a stop to them? Four Ninja in this case. Four Teenaged Ninja. Teenaged /mutant/ ninja. Okay, teenaged mutant ninja turtles.
Turtle-riffic.
Enchanced Strength : A regular turtle can pull five times its own body weight. Leonardo (along with his brothers) has been mutated and received years of training under a grand master of martial arts. As a result, his strength is many times that of a normal human.
Enhanced Speed : Turtle he may be, but slow he isn't. He can run fast enough to avoid laser fire or even avoid bullets. Equally, he can move things at an enhanced speed, such as his twin katana.
Ninjutsu : He is a master ninja and very agile, able to leap over rooftops and avoid attacks from enemies, including gunfire, weapons and punches and kicks. Leonardo takes his training seriously, more so than his brothers, and his skill rivals that of grandmasters, be that unarmed, or using his twin katanas.
Stealth : As a ninja, Leonardo has not just been trained in the art of stealth, he no longer even needs to think about it. He can hide in shadows, sneak around without being detected and move around what may seem sheer impossible surfaces. Given the slightest of opportunities, he will vanish from sight.
Shell : Although far from his humble beginnings as a normal turtle, Leonardo retains the ability to retreat into his shell, which can stand up to a surprising amount of damage.
Philosophy and Leadership : Of all his brothers, Leonardo is certainly the most spiritual and spends time contemplating ancient wisdoms. This isn't time idly spent, as they more often than not form a basis for his leadership decisions, keeping his family safe while getting the job done. He is an ardent student of Confucius, Lao Tze and Sun Tzu.
Stamina and Endurance : Due to his ninjutsu training, he has a great deal of stamina. This allows him to keep up with his training and outlast the majority of his opponents.
Pop Culture : Although Leo can often be found watching the news or other factional broadcasts, he enjoys the lighter side of entertainment as well, especially science fiction. He'll happily match his encyclopedic knowledge of his favored fandoms with all comers.
Family : They may quibble, they may argue, they may occasionally end up fighting each other, but the four turtles are brothers. They have his back and he has theirs, guided by the wisdom of his adoptive father, Master Splinter.
Gear : Although scavenged and re-purposed, Leonardo carries a full complement of tools of his particular trade. It just so happens that he's a ninja, so those tools are distinctly specialized. From grappling hooks, climbing hooks to his well maintained pair of Katanas (and many more bits and bobs), Leonardo is as well equipped as the mythical ninja of old.
Hideout : It may be in a sewer, but don't knock it. The TMNT have made their hideout into something worth living in, all the way down to lighting, heating, full living amenities, TV and internet. Rent free, no neighbors and no building codes to adhere to, there's quite a few people in Manhattan who wouldn't mind trading. On top of being secluded, its connection to the sewers makes it very easy to get around undetected.
Leonardo is the leader of the group, since someone has to be when Master Splinter isn't around to guide them. This has made him far more serious than his brothers, which leads to clashes every now and again. Still, he feels a great responsibility towards them and would do whatever it takes to keep his family safe. His habit of training harder than the others comes from this sense of responsibility, but this has also left him with an impossible aspiration to be the perfect leader, the perfect hero. Leonardo often sets himself impossible goals, and it usually takes his brothers to point out that adequate will do just as well… Although seeming to be far too serious for his age, he knows how to relax when the job is done, and how to smooth over any arguments or rivalries that might spring up between his brothers. He considers Master Splinter to be his father, his mentor and his absolute role model.
Enemies : The Ninja Turtles have amassed a great number of enemies over a short period of time, most notably the Foot Clan and its leader. While they may be formidable as a team, the entire fighting strength of the TMNT consists of four turtles and one rat, which means that all their serious enemies could easily wipe them out in a decisive attack. For that reason, it is absolutely imperative that their hideout in the sewers remains secret.
Reversible Mutation : Although he has come far from his humble beginnings in the pet store, Leonardo's mutation can be undone. The right serum could send him straight back to being a regular turtle.
Physiology : Though mutated, Leonardo is still a turtle. This means he is cold blooded and cannot produce his own body heat, which can become a serious problem in colder temperatures.
Burden of Command : While he loves his brothers, Leonardo is also their leader which can occasionally put a strain on his relationship with them. On top of the usual sibling rivalries that are common, he must also face the reality that he is responsible for their safety and even their survival. This leads him to be far more serious than his brothers, and often Leo feels isolated in the burden of responsibility he carries.