Friends and Family[]
All of the Turtles: Leo is the leader of his brothers. And he likes to be the one on the team who is not afraid of anything. That's why Raph refers to him as "fearless leader" sometimes. Leo is very protective of his younger siblings, and doesn't want anything to happen to them. As shown in the last episode of season one, Leonardo would even risk his life to save his brothers.
Master Splinter: Leo loves to train, spar, and learn more about ninjutsu. He likes to train with his sensei, and father; Master Splinter. Splinter taught him everything he knows about ninjutsu, and Leo respects him very much. Out of all the turtles, Leo has the closest relationship with his father, very often going to him for advice. Like his brothers, Leo is very protective of his father as well. Leo was the first turtle to call Splinter father in a long time.
Raphael: Leo and Raph have a very brotherly, competitive relationship. Raph is jealous of Leo due to his leadership title and he challenges him leading them into arguments. In The New Girl In Town Leo lets Raph lead after an argument so Raph can get a taste of how hard it actually is. Leo sometimes can show off. They always sweat against each other in video games and push each other to the max during training. Raphael would usually instigate the arguments.
Despite all of this, they both trust each other more than their other brothers as they often tend to discuss things privately away from them such as Leo opening up with Raph about Karai. They also care very deeply for one another. immensely care for each other. Within the Woods is a primary example of this. When Leo is unconscious from his injuries from the season 2 finale The Invasion, Raph is seen watching Leo everyday in the bathroom hoping he will wake up. When Leo regains conscious Raph helps Leo train and gain his confidence. When Raph gets mutated into a plant by the Creep and Donnie had to extract Creep's mutagen off of him, Leo never left Raph's side.
Donatello: Leo also has a brotherly relationship with Donnie. They get along usually, but they get into arguments at times. In missions, Leo (and Raph) would sometimes rush Donnie in technology especially when their lives are at risk and Donnie would erupt. Like Raph, Donnie can sometimes challenge Leo's leadership and authority.
In The Invasion, Leo and Donnie are seen arguing over how to resolve the Kraang invasion. Donnie thinks they can defeat the Kraang with the Turtle Mech despite the fact it is still under construction and Leo suggests they should evacuate the city for a little bit. In Heart of Evil, Donnie's vengeance against Don Vizioso has endangered the mission leading Donnie to argue with Leo once again. Leo also can be hard on Donnie when it comes to science also causing Donnie to lash out. Despite this, they work well together and Mikey would call them "Team Brainiac".
Michelangelo: Leo and Mikey don't talk to each other that often, but they still have a really good relationship with each other. Like Raph and Donnie, Leo can get annoyed with Mikey at times. In the episode Mazes & Mutants, Leo and Mikey worked together to come up with the game, L.A.R.P. And in the first episode of season three, when Leo finally woke up, Mikey gave him a hug and animated tears of joy came out from his eyes. Even though they don't often show it, they will always love each other no matter what comes their way. In The Pig and the Rhino Mikey begs Leo to go to the Comet Roller Coaster to find Karai. Leo grants his request and decides that he wants to go with him. The two of them work together to save their brothers and human friends from Bebop and Rocksteady. In Attack of the Mega Shredder Leo went back Mikey in an attempt to slash it open but fails and nearly got eaten himself, he looked at Mikey's Grappling Hook and gave a sad look believing he was deceased.
Karai: Leonardo is the first of the Turtles that Karai meets, and was also romantically attracted to her. While she attacks and fights with him often, she never does him any serious harm and just enjoys sparring with a worthy opponent. Leonardo, in turn, was both confused by and attracted to Karai (as shown in The New Girl In Town). She was shown to be dangerous, but Leo believed that there was some good in her and was stunned by her appearance the first time he saw her. In the first season she and Leo were basically seen as frenemies and they either flirt or actually fight. In Enemy of My Enemy, instead of wanting to fight the turtles, she decides to make a truce with the turtles in order to defeat the Kraang ship.
Once she saw Leo and the other turtles try to kill her "father" Shredder, the truce was cut and she wanted revenge on them and by the first half of season 2 she became more threatening. In Follow The Leader, Leo is angry at Karai as she wants to hurt him and his loved ones more often because she thinks Splinter took away her mom's life. Leo while fighting her, declares it's not true and her vendetta is with him- not his family. When Splinter reveals Karai is actually his first born child, that is when Leo really refuses to give up on Karai and decides to do whatever it takes get Karai with her real family. In Wormquake, Leo tried to tell her the truth about her father but she was not having it thinking it's a trick. She begins to question herself as she overhears the group talking about her being Splinter's first born. By The Wrath of Tiger Claw, Leo is elated to hear that Karai is coming around and takes her to the lair. Once Karai is at the dojo and see pictures of her and her true family, that is when Karai really does believe the truth. When Leo asks why she came down in the first place, she admits she was lying before because she wants to test to see if her brothers and father are telling her the truth and Tiger Claw is tracking her just in case. Leo forgives her. During the fight with Tiger Claw in the cold room, Karai refuses to kill Leo when Tiger Claw asked her to do so and fights Tiger Claw instead. When Tiger Claw runs off with her Leo wants to go after him, but Mikey was badly hurt so he holds back.
When the turtles find shredder's helmet in The Legend of the Kuro Kabuto, Leo wants to make a trade with Shredder which was Shredder's helmet for Karai. Unfortunately Shredder refuses to do his part of the deal. In Vengeance is Mine, Leo was willing to go behind Splinters back in order to save Karai again. After they saved her and brought her back to the lair, Leo blushed when Mikey embarrassed him in front of Karai during training. A while later at night, Karai attempted to go attack Shredder alone after knowing the truth about her real father. Nonetheless, Leo caught up and insisted he should come with her. Karai refused as it was not his quarrel with the Shredder. Leo kept persisting. To stop the argument, she got up close and acted like she was going to kiss him, only to throw blinding gas onto his stupefied face and bolt. Leo woke up only to discover she was gone, and he gathered the rest of the team to save her at Shredder's lair. He saw that she was trapped there in a cage vastly above mutagen and decided to make a leap for it. Karai gave him a thanks as he was picking the lock, and Leo smiled. All of a sudden, Shredder lunged toward the cage and swung his blades at Leo. Unfortunately he cuts the chains to Karai's cage by accident and she falls into the mutagen.
Transformed into a serpent, she slitherered out of the building, inaudible to Leo's calls for her to stop. Leo blamed himself for what happened to Karai after that. He immediately consumed a dark, brooding personality because of the incident. There was also no clue if she can still remember who Leo was due to her mutation. In Serpent Hunt they find Karai wondering the desolate Kraang-filled streets as a snake. When Leo tried talking to her, she couldn't understand anything; Donnie explains she's becoming more snake-like every minute. Leo ignores him and begs for her for communication. Next, it looked like Karai about to hurt him but instead comes to eye to eye contact, saying his name. She then converts herself into human form. "Help me," she said in a hoarse voice. Leo offerered to take her back to the lair but she claimed she's was too dangerous. Later on during the fight between the turtles and Shredder/Ivan Steranko, Leo breaks Karai free and tells her to leave before they catch her again. She rejects him only to point east and say "Comet", giving Leo a hint where she could be.
By Seasons 4 and 5, although Karai rebuilds the Foot Clan to bring honor and destroy the Shredder, she gradually opens up and allies herself with her real family, and she is especially open with Leo. In Broken Foot Karai still has her rebel streak as she and Leo are both seen getting into trouble together, taking down Shredder's operations and setting the money on fire, and Leo still either plays along or is the voice of reason. In addition they are showing a lot more support for one another especially in Requiem when Leo gives Karai CPR after the Shredder attacked the Mighty Mutanimals and set their lair on fire and Karai is supportive and thankful that Leo killed the Shredder in Owari. After Splinter's death, she is seen at the lair more often than before. Sometimes she is seen training with Leo at the dojo, as shown in When Worlds Collide when they were sparring Karai proudly claims "Just beause I like you doesn't mean I'll go easy on you Leo" and Leo smiles widely in response. Leo can also imitate Karai as also seen in When Worlds Collide when Raph is seen hugging Mona Lisa in front of the whole team in a rooftop, Karai states "Yikes, I can't believe Raph has the hots for a big newt!" Leo replies "I know, right, he's got weird taste" with the same tone and body language Karai gives.
April O'Neil: Leo and April barely talk to each other, but they are still close friends. As seen in New Girl In Town, Leo went to April's apartment balcony to tell her about how had met Karai that night. Once Leo told her that Karai was part of the Foot Clan, April pulled out a newspaper and slapped him. And she kept yelling at him, about how he shouldn't be hanging out with her, because she's in the Foot Clan, but her view of this changes long after when she learns that Karai was Splinter's very daughter.
In Eyes of the Chimera, the rest of the team (Donnie, Mikey, Raph, Casey) got kindapped by a wild, giant bird-like mutant called the Chimera, leaving April and Leo behind at the farm house. Leo was hiding because of his disabled leg, and April was under the shelter of the barn, blind; she was seeing through the eyes of the Chimera on account of Donnie's invention made for her telepathic powers. April commands that both of them should search for the rest of the team, but Leo declines gloomily. He complains that with his broken leg and with her blind eyes, they'll never make it. But with enough begging, April and Leo (who carried a walking staff) end up limping towards the high mountain where the Chimera's nest is. At one point of their trek, April could feel the hunger of the Chimera. The redhead loses her mind quickly, trying to bite Leo while making noisy caws. She instantly regains her conciousness back as a human when Leo dropped his staff and shook her by the shoulders. With great joy Leo then realised he could stand without the staff.
At one point, the Chimera captures them but they manage to ride onto its back. As the Chimera was careening into the mountain's direction unsteadily, Leo commanded for April to jump since she was blind. "Jump? Are you crazy?" she had screamed doubtfully. Right before the Chimera crashed down the rocks a mountain cave, Leo grabbed April by the shoulders and legs, leaping off the big mutant.
Near the end of the episode, Leo was helping April up some mountain rocks when he thanked her. She asked him what for, and he answered that he'd still be at the barn if not for her, also that she really inspired him. April found this kind and they hugged.
Casey Jones: Just like April, Leo and Casey don't communicate that often. But in the episode, A Foot Too Big, Leo and Casey were the barn laughing together about the jokes they were making about Bigfoot. And in The Invasion, April, Donnie, Raph, and Mikey, went down to the sewers to look for sensei, while Casey stayed with Leo in the van went out to look for his family. But, Leo was unconsious so he has no memory of when that had happened.
Enemies[]
Shredder: Out of all of the turtles, Leonardo has had the most interactions with Shredder; each of them highly antagonistic and adversarial. In Wormquake!, Shredder taunts him to watch Splinter die when he had Tiger Claw capture and poison the ninjitsu master, also telling him that it will be the last fight he ever sees. In The Legend of the Kuro Kabuto, Leo made a deal with Shredder involving the exchanging of Karai (which turned out to be a bomb) for the Kuro Kabuto, but Shredder attacks him, refusing to let the young ninja leave without a fight. In The Invasion, when Shredder learns that Leo has been separated from his brothers, Shredder tracks him down and sends Foot-bots after him, sadistically wanting to "see him suffer" before killing him. During this time, he claims that Leo is foolish and overconfident due to his youth. When Leo defeats all of Shredder's lieutenants, Shredder himself knocks him unconscious and has him taken away. In Serpent Hunt, Leo briefly argues with Shredder about his "parentage" over Karai. Recently, in Casey Jones VS. The Underworld, as Leo engages Shredder in another ferocious battle at the Auman Chemicals factory, he claims that he is going to end Shredder once and for all. Because of this, Shredder may despise Leo more than any of the other turtles while Leo may show the greatest amount of disdain for Shredder compared to all of his other enemies. Leo even killed the Shredder to avenge his father in Owari.
Tiger Claw: Since the villain's first appearance in The Manhattan Project, Leo has frequently encountered Tiger Claw and engaged him in combat both with and without his brothers. In The Manhattan Project, Leo was pinned from behind by Tiger Claw when he tried to rescue Splinter and before he attempted to draw his weapons on Shredder, only to escape his grip due to the arrival of Raphael and Michelangelo. That same episode, Leo briefly fought with Tiger Claw (even managing to surprise him with a hidden weapon in one of his Katana) and managed to kick him away from Splinter when he had the superior ninja master pinned. In The Wrath of Tiger Claw, after Leo was defeated along with his brothers, Tiger Claw planned to finish him off first, but was willing to let Karai do it instead. In The Invasion, though tired and weakened from fighting Kraang and Foot Bots, Leo engaged Tiger Claw (along with Rahzar and Fishface) and actually managed to defeat him with one kick.
Stockman-Fly: Leo tends to forget Stockman's name (examples: "Dexter Backsman", "Daxter Boxman", "Blister Speckboy", etc.)
Fungus Humungous: Leo proved highly susceptible to the spores of Fungus Humungous, which brought out his fear of failing his team as a leader. Despite this fear, he faced it and defeated the villain by exposing it to sunlight.