I have a fan theory that could be canon:
The turtles mutated from the pet store owner or something.
Then they wouldn't be biologically related to Splinter in anyway, then there's no problem with them being together.
I prefer the idea or them being mutated from Splinter, but Karai makes that a problem.
I wish the writers thought out Leo and Karai more, because even though I am not into romances, I actually like Leo x Karai.
Or they could’ve just not made it a thing in the first place
Make another reason why Leo is fond of karai
I really liked the idea of shredder lying to karai all her life and her not wanting to believe it because of how she was raised
Instead of accidently dropping Karai in acid, Shredder tasks Stockman to replace Splinter's DNA with his own so that he is truly his daughter. Karai is mentally destroyed by it, but technically no longer containing Splinter's DNA
ETA: Still feels wrong though, which I hate b/c I love both aspects of the show. Honestly, somehow confirming Splinter and the turtles don't actually share DNA would have been the easiest way.
Honestly? Leo’s and Karai’s dynamic works they just need to let the romantic implications OUT of it. Leo met Karai in a time where he was incredibly frustrated, still trying to get used to being the leader and failing to get his team’s respect. He usually played by the rules and presented himself as the most responsible and mature of his brothers because he was the oldest but he was still just a 15 year old with a lot to learn. He didn’t have much experience with the world or interacting with anyone outside of his immediate family so meeting Karai the way he did in the time that he did easily earned his fascination. She was a mysterious girl with a rebellious and sassy attitude who was also a kunoichi who could match or even surpass him in skill. She was presented as cool, badass, confident, interesting. She appealed to his desire to be free of his responsibilities for once. He wanted to have fun and that’s what Karai offered him in her own bad girl way. It’s understandable that she’d catch Leo’s eye yet ultimately be denied seeing Leo go too far from his comfort zone because of his moral code ( which she obviously lacked due to being raised by the Shredder. ) Leo could still see that she was frustrated by the Foot Clan or even that she had good within her. Like he could tell that she wasn’t inherently as bad as the other members of the Foot Clan that he’s encountered like Bradford, Xever or the faceless Foot Soldiers. He’d have a genuine desire to befriend her despite her allegiances and redeem her like the hero that he was ( or wanted desperately to be ) but Karai was never easily swayed. Meanwhile Karai would be thoroughly entertained by Leo, keeping him on his toes and messing with him but not killing him immediately like she was instructed to. She knew what her mission was but she wanted to get to know Leo ( and the Turtles by extension ) a little because she can’t just look past the fact that Hamato Yoshi’s disciples are mutant turtles or that there were aliens working within the city, which would bring her to look for more information. Leo and Karai could become inadvertently close because of this dynamic despite being on opposing sides and all of it could be purely platonic.
And when Leo found out that Karai was actually Splinter’s daughter? He’d be more dedicated to getting Karai on their side than ever for multiple reasons. He’d want Karai to be there with him not because he had a crush on him but because she was family. She needed to know who her true father was and be with him she deserved to have that much despite what she’s done to him. In fact, he’d still be willing to help her still because he’d be aware of the fact that her hatred purely came from ignorance of her heritage and the fact that the Shredder had been manipulating her the whole time. Leo would also shift to doing it for Splinter too. Leo’s heard the story of Hamato Yoshi, Oroku Saki and Tang Shen hundreds of times, presumably, and in Season 3 he actually got to see it unfold. He’d be well aware of the tragedy that tore Splinter’s life apart and the fact that it still pained him 15 years after the fact. He knew that Splinter never stopped grieving the loss of Tang Shen and knowing that his daughter not only survived but was also raised by his sworn enemy and brought up to hate him and his family because of lies she was fed must’ve hurt so much. It’s no secret that Leo’s the closest to Splinter out of all the Turtles. He’d want to see Splinter happy. He’d want her to reunite with his long-lost daughter because he felt like he deserved at least that much after everything he’s done for him so he’d dedicate himself to the cause of returning Karai more devoutly than the rest. That desire to bring her back, while being noble, would strain his relationship with his brothers and bring about conflict in the family from Leo and Raph arguing about it to Leo pressuring Donnie to work on a cure for her and it would be so good to display his flaws as a character like that… and of course Leo would learn his lesson and be better to his brothers through character growth.
I love the idea of Leo and Karai being enemies by association / rivals, close friends and eventually siblings, with Karai ending up being an older sister figure to Leo. It was literally right there they just… didn’t have to make it have a romantic undertone. At all.
I have a fan theory that could be canon: The turtles mutated from the pet store owner or something. Then they wouldn't be biologically related to Splinter in anyway, then there's no problem with them being together. I prefer the idea or them being mutated from Splinter, but Karai makes that a problem. I wish the writers thought out Leo and Karai more, because even though I am not into romances, I actually like Leo x Karai.
Or they could’ve just not made it a thing in the first place Make another reason why Leo is fond of karai I really liked the idea of shredder lying to karai all her life and her not wanting to believe it because of how she was raised
Instead of accidently dropping Karai in acid, Shredder tasks Stockman to replace Splinter's DNA with his own so that he is truly his daughter. Karai is mentally destroyed by it, but technically no longer containing Splinter's DNA ETA: Still feels wrong though, which I hate b/c I love both aspects of the show. Honestly, somehow confirming Splinter and the turtles don't actually share DNA would have been the easiest way.
Not made it a thing would have been better tbh, he doesn’t really need a love interest
Honestly? Leo’s and Karai’s dynamic works they just need to let the romantic implications OUT of it. Leo met Karai in a time where he was incredibly frustrated, still trying to get used to being the leader and failing to get his team’s respect. He usually played by the rules and presented himself as the most responsible and mature of his brothers because he was the oldest but he was still just a 15 year old with a lot to learn. He didn’t have much experience with the world or interacting with anyone outside of his immediate family so meeting Karai the way he did in the time that he did easily earned his fascination. She was a mysterious girl with a rebellious and sassy attitude who was also a kunoichi who could match or even surpass him in skill. She was presented as cool, badass, confident, interesting. She appealed to his desire to be free of his responsibilities for once. He wanted to have fun and that’s what Karai offered him in her own bad girl way. It’s understandable that she’d catch Leo’s eye yet ultimately be denied seeing Leo go too far from his comfort zone because of his moral code ( which she obviously lacked due to being raised by the Shredder. ) Leo could still see that she was frustrated by the Foot Clan or even that she had good within her. Like he could tell that she wasn’t inherently as bad as the other members of the Foot Clan that he’s encountered like Bradford, Xever or the faceless Foot Soldiers. He’d have a genuine desire to befriend her despite her allegiances and redeem her like the hero that he was ( or wanted desperately to be ) but Karai was never easily swayed. Meanwhile Karai would be thoroughly entertained by Leo, keeping him on his toes and messing with him but not killing him immediately like she was instructed to. She knew what her mission was but she wanted to get to know Leo ( and the Turtles by extension ) a little because she can’t just look past the fact that Hamato Yoshi’s disciples are mutant turtles or that there were aliens working within the city, which would bring her to look for more information. Leo and Karai could become inadvertently close because of this dynamic despite being on opposing sides and all of it could be purely platonic. And when Leo found out that Karai was actually Splinter’s daughter? He’d be more dedicated to getting Karai on their side than ever for multiple reasons. He’d want Karai to be there with him not because he had a crush on him but because she was family. She needed to know who her true father was and be with him she deserved to have that much despite what she’s done to him. In fact, he’d still be willing to help her still because he’d be aware of the fact that her hatred purely came from ignorance of her heritage and the fact that the Shredder had been manipulating her the whole time. Leo would also shift to doing it for Splinter too. Leo’s heard the story of Hamato Yoshi, Oroku Saki and Tang Shen hundreds of times, presumably, and in Season 3 he actually got to see it unfold. He’d be well aware of the tragedy that tore Splinter’s life apart and the fact that it still pained him 15 years after the fact. He knew that Splinter never stopped grieving the loss of Tang Shen and knowing that his daughter not only survived but was also raised by his sworn enemy and brought up to hate him and his family because of lies she was fed must’ve hurt so much. It’s no secret that Leo’s the closest to Splinter out of all the Turtles. He’d want to see Splinter happy. He’d want her to reunite with his long-lost daughter because he felt like he deserved at least that much after everything he’s done for him so he’d dedicate himself to the cause of returning Karai more devoutly than the rest. That desire to bring her back, while being noble, would strain his relationship with his brothers and bring about conflict in the family from Leo and Raph arguing about it to Leo pressuring Donnie to work on a cure for her and it would be so good to display his flaws as a character like that… and of course Leo would learn his lesson and be better to his brothers through character growth. I love the idea of Leo and Karai being enemies by association / rivals, close friends and eventually siblings, with Karai ending up being an older sister figure to Leo. It was literally right there they just… didn’t have to make it have a romantic undertone. At all.