Some TMNT stuff really isn't for little kids.
In the IDW continuity, Leonardo, just like his brothers, is a reincarnation of one of the sons of Hamato Yoshi that in medieval Japan were murdered by Oroku Saki. He began his new life in the present as a water turtle hatchling in the laboratory of StockGen Research, Inc., Baxter Stockman's genetics laboratory where he served as a guinea pig for an experiment to bred naturally armored super-soldiers. Some ninja tried to seize the Turtles and various other research results from the laboratory, including a super soldier mutagen. The intervention of his also reborn father, now a lab rat, resulted in the turtles and Splinter being were wetted with a chemical cocktail which eventually made them mutants.
In the fifteen months that followed, Leonardo was extensively trained by Splinter in ninjutsu, with twin katanas as his personal weapons. Along with Donatello and Michelangelo, he searched the alleys and rooftops of New York for their fourth brother, Raphael, who had been separated from them during the mutation process. Even when the quest seemed hopeless, Leonardo steadfastly refused to give up. Shortly after finding him, Leonardo was given a blue mask as a sign of his individuality.
Unfortunately their reunited family did not have peace for long, as Old Hob abducted Splinter with the aid of an army of M.O.U.S.E.R.S robots. Leonardo went out in search of his missing father, only to be attacked by a vast group of Foot ninja and defeated in combat. He and his brothers then infiltrated StockGen with the help of April O'Neil, only to discover that Splinter had been taken by a group of ninja.
The Turtles regrouped at the O'Neils' closed-down antique shop, where Leonardo clashed once again with Donatello about whether the ninja who attacked him were the Foot Clan. They were subsequently shown to one of the Foot Clan's hideouts by Angel Bridge and the Purple Dragons, and Leonardo led them in reclaiming Splinter.
Shortly thereafter, the Turtles encountered the "Slash monster," and Leonardo attempted to end things peacefully with the larger mutant turtle. Unfortunately, he accidentally stabbed Slash with one of his swords, and was deeply saddened by the belief that he had killed him. When the Turtles were transported to Dimension X, he coordinated plans with Commander Dask to infiltrate the Neutrino royal castle and rescue the King and Queen, which brought him into direct conflict with the malevolent General Krang.
Leonardo was captured by the Foot Clan shortly thereafter, and was brainwashed by the witch Kitsune into becoming a faithful follower of Shredder. Believing his brothers dead and his father cruel and faithless, Leonardo was made the chunin of the Foot Clan and was instrumental in bringing down the Savate ninja organization. However, he was still struggling subconsciously against his brainwashing, with the help of his dead mother Tang Shen. Raphael eventually managed to break through his brother's brainwashing, and he was reclaimed by his family before they fled to Northampton.
In Northampton, Leonardo spent all of his time in the woods by a lakeside, where he hallucinated and pleaded with his mother to help him. Though initially he was hostile towards Raphael and Splinter, he gradually began to return to his old self, and donned a new blue mask as a sign of reconnecting to his old life.
Upon returning to New York, Leonardo found himself facing another powerful being that attempted to warp his mind: the Rat King. The trickster attempted to turn Splinter and Leonardo against one another, using the recent rift between them as ammunition, but Leonardo had worked past his doubts about his father and refused to be tricked.
While time-traveling with his brothers, Leonardo found himself encountering the previous incarnations of his family in feudal Japan, including his mother Tang Shen and his own younger self. He attempted to change the past by killing Oroku Saki before he could murder his family, but Raphael stopped him. He also was accidentally transported to a parallel world, where he bonded with the Ghostbuster Winston Zeddemore.
Leonardo found himself in the middle of a familial dispute when Donatello and Splinter disagreed on whether they should be giving priority attention to the Technodrome or to Shredder. Behind Splinter's back, he hatched a plan with Donatello to deal with both of their problems simultaneously, attacking and disabling the Technodrome while pitting Shredder against Krang. He led the charge to Burnow Island and successfully defeated Krang, saving the world from the Technodrome's terraforming. Upon returning, he learned that Donatello had nearly been murdered by Rocksteady and Bebop, and attempted to keep Raphael and Michelangelo on an even keel.
After being attacked once again by the M.O.U.S.E.R. robots and flyborgs , Splinter entered his family into the Gauntlet, an ancient ritual that would end the bloody conflict between them and the Foot. Leonardo was assigned the hawk mutant Koya as an opponent, and he successfully defeated her by slashing off her flight feathers, crippling her permanently. He participated in the final fight against Shredder along with his brothers, with his father as the victor.
When Splinter accepted leadership of the Foot Clan, Leonardo once again became the clan's chunin, though it was not a role he was comfortable with. Much of his duty was to shore up the Foot's reputation in the wake of their recent defeats, and to stop other criminal elements that might try to take over. He also took over the training of new Foot recruits, encouraging flexibility and versatility in their martial arts experience.
He set aside his Foot duties for a journey to Burnow Island, but the trip proved to be a difficult one as they encountered the hostile mutant Leatherhead, who nearly suffocated and crushed Michelangelo as a way of making Donatello teleport him to New York. And upon returning to the Foot Clan headquarters, he was horrified to see Kitsune threatening his father's life. Leonardo was the only one of the Turtles to not be ensnared by the witch's powers in the fight that ensued, having gained some immunity to them.
Despite this event, the fractures in his family grew more intense as the Foot Clan prepared to battle the Street Phantoms, and Leonardo found himself increasingly alienated from his father and unaware of what he was planning. This came to a head with the execution of Darius Dun, which drove away his three brothers and caused Splinter to confide in him that he had been deliberately driving his sons away for their own safety. Now it was Leonardo's duty to lead them. This broke Leo's heart, but he understands Splinter's advice, so he parted ways with his father, saying his goodbye and leaves.
This new responsibility and the distance from Splinter weighed on Leonardo's mind in the days that followed, as he had difficulty convincing his brothers to follow him. However, he still led the Clan Hamato effectively during clashes with the Earth Protection Force, and during the infiltration of the Pantheon Family Reunion.
He returned to Dimension X with his brothers in the Trial of Krang story arc, in which the Turtles set out to find assorted witnesses to Krang's prior atrocities in order to convict him of war crimes. During the trial, he participated in the defense of the Neutrino homeworld from the Malignoid swarm. Shortly afterwards, he and his brothers were pursued by the demonic Collectors, and he was forced to flee with Winston Zeddemore on an interdimensional journey.
Leonardo's relationship with Splinter became more strained upon his return to Earth, since the Foot Clan was attempting to exterminate the Triceratons. He tried to convince Splinter that there was another solution to the conflict, but Splinter refused to believe this, and even attempted to imprison the Turtles for their own safety. Upon escaping, Leonardo openly sided Clan Hamato with the Triceratons, and even fought his beloved father to defend them.
After the invasion was over, Leonardo experienced a precognitive vision of the Rat King, and led his brothers in a quest to thwart their enemy. Victory was eventually achieved when Leonardo fought the Rat King on the astral plane. After this, Leonardo made further ventures onto the astral plane to prepare for further battles against the Pantheon, but came into conflict with Kitsune there.
While on a brief vacation with his brothers in Northampton, Leo was waylaid by his old nemesis Koya, who attempted to kill him. However, Leo's calm demeanor and unexpected apology caused Koya to draw back on her revenge, and allow him to live. He reconnected with Karai and offered to convince Splinter to let her have leadership of the Foot Clan peacefully, so he and his brothers could have their father back. Afterwards, he enjoyed a day of peace and relaxation with his brothers before they returned home.
Leo, along with his family (sans Splinter) and friends celebrating Christmas. However, when Splinter doesn't want to celebrate Christmas, he was visited by three ghosts of Christmas. Splinter invited (particularly captured his sons and allies) to celebrate their Christmas night, making Leo and his brothers happy.
Leo was present at Slash's funeral, mourning his fallen ally.
Like other incarnations of Leonardo, the IDW Leonardo is very loyal to Master Splinter, the leader of the group, and the oldest of the four turtles. He has also been shown to be the most calm and cool under pressure. However, his early arguments with Donatello reveal that he has inherited Splinter's temper when provoked, though one less volatile than Raphael's.
He also takes his training very seriously, and strives to be the best that he can be. Even when he and his brothers are out in the city on patrol or do training sessions, he makes sure that he and his brothers are doing their best and training as hard as possible, even when Splinter isn't with them.
As with many other incarnations of the character, he tends to take the weight of the world on his shoulders, expressing self-recrimination for any mistakes he perceives himself to have made, such as when he blamed himself for Splinter's execution of Darius Dun. He also tends to become frustrated when he believes his brothers are not listening to him.
While he usually tends to play by the rules and listen to what Splinter orders, Leo has been shown to have a rebellious streak. This is shown in Prelude to Vengeance, where he recklessly engaged the Purple Dragons in a street fight, even though it meant that the Turtles were exposing themselves on a public street easily visible to humans, against Splinter's wishes.
Leonardo is also the most spiritually-minded of the four Turtles, even characterizing himself as "the one who believes in miracles." Despite initially worrying that his visions of Tang Shen meant he was mentally ill, he easily accepted the idea of his mother's spirit visiting him when Splinter reassured him. He also was the Turtle most inclined to accept the idea that they were reincarnated, and shared Splinter's belief that cosmic destiny had guided their lives.
After being captured by Foot Clan and brainwashed by Kitsune, he becomes loyal to the Foot and Shredder. Under Kitsune's control, he becomes more like Shredder - sinister, cold, calculating, ruthless, cruel and murderous, showing no remorse for attacking Splinter and his brothers, or even cutting Victor's hand. However, he was freed from brainwashing.
- It is revealed in issue #32 that Leonardo is right-handed.
- He named his swords Musashi and Kamiizumi.
- Leonardo is the first of his brothers to be able to remember any of their family's past lives, much of which seems to revolve around his mother and her death.
- Leo is right-handed, as in one issue when Koya injures Leo's left arm he states that he's lucky it wasn't his throwing arm that was injured, and then throws shurikens at Koya using his right arm, thus majing him right-handed.