Disclaimer: I don't own TMNT, but the first few lines are from the actual episode. I also didn't come up with the idea of Karai being Leo's "sweetheart"; I read it on Wikipedia, in their quotes for the Monsters vs Mutants arc. Aaand big spoilers for the Wanted: Bebop and Rocksteady arc. Abbreviations = 2012 turtles, full name = 1987 turtles. Hope you enjoy :)

"Then we stop 'em before they get a chance!" declared Raphael, clenching his hand into a fist.

Suddenly, a cheerful and strangely familiar beeping filled the air. Everyone glanced around uncertainly, their eyes ultimately falling on Leo as he pulled out his phone. "Hold up... Karai?" He held the device to his ear.

"Leo," a girl's voice echoed in the silence, "you're not gonna believe this. I'm at the water front; I followed Bebop and Rocksteady, they've got a bunch of weird Foot robots with them. They're targeting a secret military warehouse, and I'm pretty sure it belongs to the Earth Protection Force."

Leo set his jaw. "Don't make a move yet. We'll be there soon." He hung up the phone and winced. "Okay we need to get to the water front straight away, she never listens to me. Let's go, team!"

Eight turtles and two confused humans rushed out of the lair, melting silently into the sewers. They moved quickly, Leo hastily briefing the team as they went. His brothers and their absurdly young April and Casey all seemed to understand, nodding calmly, but their dimensional visitors were feeling left in the dark. Michelangelo took it upon himself to find some answers, so he dropped his pace, falling into line beside his orange-banded counterpart.

"Hey, Mikey?" He asked quietly. "We're going to help that girl, right?"

"Yeah, Karai." Suddenly, his big blue eyes widened. "Wait, is Karai still a bad guy where you're from?"

"Karai?" Michelangelo frowned, straining to remember but coming up short. "Don't think we've ever met a Karai before."

"WHAT?!"

If Mikey's exclamation hadn't already captured the group's attention, his sudden screeching to a halt would have done. He stared at his counterpart in astonishment, mouth floundering for words and never finding any, even as his own brothers were getting frustrated. Donnie sighed. "What's wrong, Mikey?"

"We don't have time for this," Leo reminded him.

Finally finding the ability to speak, Mikey gasped out, "Dudes, in their universe, they don't have a Karai!"

His brothers were silent, as were April and Casey. Leonardo was about to ask what the big deal was when all five of them cried out a shocked, "What?!"

Leo stepped up to his counterpart. "You guys don't have a Karai? You've never met her, not even once?"

Rubbing the back of his neck sheepishly, Leonardo shrugged. "Well... No."

Raph scoffed. "Lucky you, she's been a pain in our a-"

"Raph, this is important!" Leo scolded. "How can they not have a Karai?! Wasn't there a Miwa in your universe? We might have something important to tell you."

"Miwa's not ringing any bells either," Raphael replied, arms folded. "Guess they just didn't translate between dimensions."

"B-but... But how is that possible? Karai's a part of our family, she's... she's important! She's a huge part of our lives! How can she just... not exist for you?" The blue-banded turtle was beginning to panic now, his face paling to a sickly shade of green.

Raph sighed, clamping a hand on his brother's shoulder. "Come on man, we can't freak out about this right now. Our Karai exists, and she needs backup. We can work out whatever's up with their wacky universe when we've beaten Bebop and Rocksteady into the ground, okay?" He chuckled, and added as an afterthought, "Besides, a world without Karai could still exist. You'd just get less action."

Now a brilliant shade of crimson, Leo whipped around and elbowed Raph in the plastron, painfully embarrassed. Casey was doubled over with laughter and Donnie and Mikey were snickering too. Only April seemed to sympathise with his plight, chastising Raph rather fruitlessly. Their visitors, however, just seemed baffled.

Donatello pursed his lips in thought. "What do you mean, less action? Does your Leonardo fight this Karai girl a lot?"

"Well, yeah," Raph smirked again, already moving out of Leo's range, "but I meant a different kind of action." He waggled the eyebrows he didn't have, and it suddenly seemed to dawn on the other turtles what he meant.

"W-wait..." Leonardo stammered, clearing his throat. "Is this Karai girl you're... you're sweetheart?"

Another burst of laughter from Casey, this time with Mikey's contribution. Raph himself was almost choking on his own mirth. "Sweetheart? Nah. Karai's not sweet, and she may not even have a heart. She does have Leo whipped, though."

"They're both in denial but they're totally dating," Mikey added eagerly, still giggling away to himself.

"Guys," sighed Leo, his voice threatening to break. "Do we have to do this now?" When the looks on their faces made it very apparent that yes, they did, he took a deep breath to steady himself and groaned. "We're not official, we're just kinda... a thing."

"That... sounds a lot like she's your sweetheart," Raphael informed him. "Not your girlfriend, but still yours?"

"She's not 'mine'! I don't own her!" Leo protested frantically, before letting out another sigh. "Can we please discuss this later? We have to go!"

He must have had some good karma stored up for all those times he had saved the world, because he was thankfully granted peace, for now. The team was on the move again. As disgusting as the sewers were, it was a blessed relief compared to the thought of a full-scale interrogation from two sets of brothers. At least he knew now that he had definitely chosen a trustworthy confidant; April hadn't breathed a word of anything he had told her. That was another great mercy from above, considering how much he had blabbed to her lately. He would thank her when he got the chance.

Just when he thought he was free for now, Leonardo jogged up beside him. "Hey, this Karai girl... If you don't mind me asking, what's she like?"

"She's..." He smiled to himself at the thought. "She's kinda mean, but sweet if she lets you get that close. Really fun, too, and an amazing kunoichi. She's... she's great."

Leonardo smiled warmly. "I wish I could meet her."

"Well, you can, soon." Leo's eyes lit up with mischief. "Actually, you can meet her first! She'll think it's me, it'll be hilarious! I can't wait to see her face!"

"Y-yeah, I... I kinda meant, I wish I could meet my own Karai, back in our world. You seem... really into her." Leonardo coughed awkwardly. "But meeting yours'll be awesome too. Do we... do you love her?"

Leo paused before answering, thinking hard about a question he had long known the answer to. "... Yeah. Don't tell her, though. She'd kick my butt."

One exhausting but ultimately successful mission later, Leo and his brothers - his true brothers - traipsed back into the lair, April leaning heavily into Donnie's shoulder. She brightened up considerably when she saw Casey lying on the couch, Shinigami sat playing a video game at his side. She greeted them cheerfully, asking them questions about the mission and confirming Casey's two broken ribs. As April scolded him and Donnie set about checking Shinigami's bandaging, the young witch caught Leo's eye, subtly inclining her head towards his room. He understood before she mouthed Karai's name.

Yawning some half-hearted excuse, Leo used the chaos as a cover to slip out of the room. As expected, someone was curled up in his bed, hidden entirely by the duvet. He smiled to himself; at least she was making a habit of coming to the right place when she was upset. Shutting the door carefully behind him, Leo moved closer, climbing into the bed beside her.

Karai didn't react until he lifted the duvet to wriggle in, and even then she merely scowled. "Stop it, it's cold."

He glanced over her. She had discarded her armour, but was still wearing her thin black bodysuit. He nuzzled up to her. "Then cuddle. I'll keep you warm."

She let out a sharp laugh. "I don't cuddle, Leo," she said firmly, but she didn't resist when he pulled her into his arms.

When her cheek pressed against his neck, he realised it was damp. Uh oh. Crying was not a good sign. "Do you want to talk?" He murmured into her hair.

The word was almost inaudible, even in the silent room. "Shredder."

Leo nodded sympathetically. "You beat him. And that wasn't even our Shredder."

"Our Shredder would've pummelled me," Karai sighed softly. "I never could fight him properly."

"He trained you, that makes sense. He knew how you fought." Three thick green fingers combed through her hair. "This Shredder didn't train you, and he was so scared of you he made his robots fight you instead!" Tactfully, Leo left out the detail that almost nobody from that dimension could fight well.

Karai fell quiet after that, so he settled for just holding her instead. His counterpart's words kept running through his head, and the more he thought about it, the more fitting the term seemed. 'Girlfriend' seemed too common, too fickle to describe everything they've been through. 'Sweetheart' didn't quite encompass it either, but it didn't seem as if there was any word that could. He liked the delicate nature of the word, and the relaxed nature of it. After everything, it seemed a little bit pointless to stick themselves in the 'relationship' box; couldn't they simply enjoy what they had worked for without labelling it? Leonardo seemed to understand that, but then of course he would.

As he watched Karai nestle closer to him, he tried to imagine a world where they had never met, and found he simply couldn't. His vision blurred, and he couldn't clear it before a teardrop fell onto Karai's forehead. She blinked in confusion, peering up at him. "What?"

He hastily scrubbed the water from his eyes. "Sorry. Just thought too much."

She sighed. "That's new. You okay?"

"Yeah." He planted a quick kiss on her forehead. "I'm glad I've got you."

Karai groaned. "Don't get all sentimental on me, dork. It's sweet, but it's not my thing."

Leo smirked. "Really? You wouldn't describe yourself as a... sweetheart?"

Her face wrinkled in utter bemusement. "What are you talking about?"

The turtle shook his head fondly. "Doesn't matter, sweetheart."

"Stop calling me that. I'm not sweet and I don't have a heart."

"That's what Raph said," he commented absently, "but Leonardo and I disagreed. He wasn't impressed with being called goofy and bug-eyed, though."

"Well, that's what he was!" Karai protested stubbornly. "And why were he and Raph giving you dating advice?"

He bit his lip. "Does that mean we're dating?"

Karai froze, realising what she had said. As always though, she recovered quickly, and took it into her stride as if she had intended to say it all along. "Unless 'friends with benefits' is more your style."

"No," Leo smiled, "I like dating more. Although I vote we don't make it official to the team."

"Absolutely not," she agreed.

He tilted Karai's head up and kissed her on the lips. They had long since mastered the difficulty of their mismatched lips, after many nose-snout collisions and several awkward moments. As their mouths moved together and she pressed herself into his plastron, Leo wondered idly if they would ever say those three little words. He imagined they would, eventually. He wouldn't be able to hold them in forever. Maybe she would even return them, one day. She clearly reciprocated the feeling, he thought, as she nibbled lightly on his lower lip.

Breaking the kiss, he twisted her up in his arms and squeezed her tightly. He would have happily stayed beside her in that bed all night, but there was an excitable and victorious team in the room nearby and they wouldn't leave them in peace for long. They shared one last, long kiss, before he scooped her into his arms and carried her from the room. She kicked her way out of his embrace and left him in the dust, smirking to himself. A non-sweet sweetheart without a heart. He liked it. He liked her.