A/N: Here is part 2 to Brothers, this time with Donnie and Karai! I wasn't entirely sure about these two, because they hardly interact, but then again this is kind of an AU, so I guess it doesn't matter too much. Hope you like it!

I OWN NOTHING!

Enjoy!

Brothers Part 2: Donatello

It wasn't that Donatello didn't like her. It really wasn't. In fact, after she had firmly established that her ties with the Foot Clan had been cut completely, and after watching the way in which she interacted with Mikey and Leo (unsurprisingly enough) he had actually grown rather fond of his half-sister. But the purple clad genius, whilst he was never really one to hold a grudge for too long, just couldn't help but remember everything she had done to him and his brothers beforehand. She'd kidnapped Leonardo once over a grudge, she'd endangered April twice over some kind of jealousy, and that was only a handful of the things she'd done whilst in the Foot Clan.

But despite all of that, Donnie knew that he had to let the past go. Heck, he'd done the same for Casey, and now they were best friends...most of the time. Karai had been through just as much as the rest of them had, with being a half mutant half snake hybrid and all; what she needed now more than ever was true affection from her true family…

However, Donnie was currently struggling with that factor as he sighed heavily, and then spun around in his chair to find said snake girl sitting in a chair opposite him, one leg crossed over the other and her arms folded as she stared at him with her glowing serpent green eyes. Though her smile was innocent – despite the fangs protruding out from her lips and the occasional flick of her forked tongue – and her posture was nonchalant and seemingly sweet, Donnie was not fooled.

She was just like the rest of them. The witch had come to torment him. He was sure of it.

"What do you want, Karai?" he asked tiredly as he turned to face her completely.

Karai blinked once, her smile never wavering. "What'cha doin'?"

Donnie's expression turned flat. That sounded like something Mikey would say, and he wondered if he'd bribed her somehow jut to get her to annoy him in his stead. "I was trying to conduct some experiments on things that you probably wouldn't find interesting. Why?"

In response, Karai shrugged. "What? I can't come in to see what my dearest 'little brother' is doing?" she smirked as she uncrossed her legs as she stood up and casually made her way to stand beside Donnie, peering over his shoulders to stare at the bubbling chemicals as they frothed from their test tubes. Her green eyes lit with slight interest as she stared. "...actually...this is kinda cool," she said.

Donnie took her moment of distraction to get a good look at the girl standing beside him. She was currently wearing a long grey sweater with a big black love heart pattern stitched on the front, the sleeves nearly covering her hands due to how big it was, and thick black leggings. She wore a pair of fluffy pink bunny slippers on her feet, though they were worn from years of use and didn't fit the outfit at all, but they fit her feet quite nicely.

It had been a few weeks since she'd officially moved in with them, and since she'd discarded her old Foot Clan attire, April had been kind enough to 'donate' her some of her clothes until further notice. Most of the clothes Karai now owned included over-sized sweaters, tank tops, jeans and the like; not nearly as glamorous as Donnie had deemed someone like Karai would dress.

Donnie blinked once, his mahogany eyes bewildered. He had thought that a girl with her background and tastes would be wearing black jackets, ripped jeans and fingerless gloves with spikes on the knuckles to send a message to anyone that she was not one to be messed with. And yet, she was even wearing freaking bunny slippers, and not a word of complaint had come from her yet.

She looked nothing like the girl that they had seen as a threat for over a year. In fact, as she continued to stare mesmerized by the assortment of chemicals on his work bench, she actually looked…kinda cute. Like the innocent girl that Leo must have seen within her from the very start...

Blinking rapidly, Donnie quickly snapped himself out of it. She may have been his sister, but she was still a formidable kunoichi; he couldn't let mere appearances catch him off guard so easily -

"Do you ever relax?"

Her sudden query snapped him out of his thoughts as he looked back at her to find that she had torn her gaze away from the chemicals, and was now sifting through his scattered notes one by one, her harlequin eyes struggling to read the scruffy writing. "What?" he asked in response, to which she sent him a flat look that oddly enough reminded him of...well, him.

"I said, do you ever just relax?" she repeated, stacking the papers in order again as she set them down on the table. "I mean, I almost never see you come out of your lab. You're like a turtle Hermit or something. Don't you have anything more fun to do other than...this?" she asked, sweeping her arm to gesture the entire room.

Pouting up at his half-sister, Donnie folded his arms and turned away from her. "Actually, contrary to popular opinion, I find the study of science to be quite interesting," he announced proudly. "Why, I remember learning the basics of Chemistry when I was only eight years old. Which, I might add, I can quote from the top of my head at any given time if I so wish."

Silence.

Donnie turned back to Karai to find her staring at him blankly. Then with a soft sigh, the half mutant girl closed her eyes and shook her head as she wrapped one arm around his shoulders and brought him close to her side despite his squawk of surprise at the unexpected gesture. "Donatello," she said forlornly, stroking the purple clad turtle's head with her other hand, "you poor, poor soul."

The surprise fading away promptly, Donnie pouted again as he shrugged himself out of Karai's grip and stood from his chair. "Alright, fine then," he grunted as he folded his arms over his chest as she turned to him with her playful smirk back in place once more. "Science might not be fun for the rest of you, but there are other things that I do besides that!"

"Oh?" Karai said as she casually leaned against his desk, being careful not to disturb the chemicals as she crossed her arms. "Then what are they exactly? Not including your sad attempts to flirt with April."

Donnie growled at her for that, but quickly brushed it off as he began to list off the things he enjoyed doing. "Well, I like the occasional video game, chess, and I like to dance -"

Karai instantly perked up at that. "You dance?"

Donnie stopped as he blinked at her. "Um, well, yeah I guess," he shrugged as he scratched the back of his neck shyly. "We all dance when we feel like it. I mean, come on, who doesn't like dancing - uh oh."

He stopped once again when he noticed Karai looking directly at him, the dangerous smirk and the ominous glint in her eye that he had known from all of their previous encounters had returned. He gulped nervously as he took a step back, but she quickly closed the gap between them as she dashed forwards and grasped his large wrists in her hands.

"Well then, show me what you got, Dork-a-tello," she grinned.

Donnie rolled his eyes. Even though there was no one in the lair right now (Splinter was taking a walk, and Leo had taken April and Mikey out for a training exercise whilst Raph and Casey did their usual goofing off), Donnie was still rather conscious of the fact that he would be dancing with Karai of all people. What if some one caught them and then teased him for his awkward moves? What if Karai laughed at him? What if -?

Karai tapped his nose once, starling him out of his worries as he looked back down at her. "Stop worrying," she chided firmly as she led him out of the lab. "Try to loosen up a little bit. It's just you and me having a bit of fun for once. Jeez, you're no better than Leo."

Donnie smirked out loud at that, his shoulders no longer stiff as he allowed himself to be dragged away from his work and into the living room. "Well if you ask me, you're no better than Mikey and Raph combined," he shot back.

"Ooh, sassy smack-talking nerd," she snickered. "We should call you 'Sass-a-tello' from now on."

"Just don't throw a hissy fit when it doesn't work."

"Oh, that was bad. Like, 'Mikey' bad."

"Not as bad as you were."

Karai stopped dead in her tracks. Slowly, she turned her head to face him, pinning him under her deadly gaze. "...what do you mean by 'were'? I'm as bad as ever," she grinned.

Donnie only rolled his eyes. "Says the girl in the pink fluffy bunny slippers."

Karai pouted. "Fine, you win this round. Let's just dance already before I get bored and decide to swallow you whole."

It was only moments later before the two siblings found themselves flailing their limbs about in the most ridiculous manner as music blasted out from the stereo, laughing as they spun and jumped and sang along to the lyrics, though both voices were horribly out of tune, but they could care less. Beaming all the while, Donnie linked his arm with Karai's as the two spun around. Hearing Karai laugh like this, watching her actually enjoying herself in a way that she could have never done within the Foot Clan, made Donnie happy inside.

The song then changed to a slower one, but they realized with twin gasps of excitement that it was 'The Time of My Life'. Both of them knew what to do as the song neared its chorus.

Karai took a few steps back, grinning like a fool. Donnie readied himself, opening his arms as he returned the smile. "You ready?" he laughed. Karai nodded, and with seconds to spare before the singers reached the chorus, she ran towards Donnie, and he lifted her right off the ground with ease by her waist and held her high above his head. She let out a breathless laugh as she gawked down at Donnie, who beamed right back with his gap-tooth showing before he let her drop back into his arms.

He watched as traces of tears yet to be shed started in her eyes as she continued to laugh. This was his sister. This was a girl that deserved to feel this way all the time, despite all the odd twists and turns that her life has taken so far. This was what she wanted. And truth be told, Donnie was more than willing to share it with her, no matter how stupid it seemed.

And besides, he hadn't had this much fun in a long time.

"What a performance, I must say."

Their laughter stopped instantly at the sound of a very familiar voice, and they spun around to find Master Splinter standing there, his hands clasped behind his back as he smiled at the two of them. Behind him, Leonardo stood with his arms folded and a similar warm smile on his face, whilst Michelangelo and April were snickering like mischievous children at the baffled pair before them.

With wide eyes, Karai and Donnie shared a look.

Karai smirked.

Donnie winced and slapped a hand over his blushing face. Well, he was lucky that Raph hadn't seen it, or he'd never hear the end of it.

But if it meant making his new sister happy...well, he assumed he could deal with that. Maybe.

A/N: All of the fluff! I hope you guys liked that, cus there's more on the way!