Keep in mind I wrote this back in 2018, before the Rise of the TMNT even aired. This is all wildly out of character and I didn't really feel like rewriting it. I re-edited to change Leo's love interest to Karai, and some grammar and paragraph structure. Also I haven't seen ROTTMNT in a while, so I don't know if they ever did anything with Karai's character. So take this as a huge canon divergence.
Raphael curled his fingers into a fist as he rapped on Leonardo's door. He had been in there for most of the time since they had returned from their latest mission. The mission was somewhat of a success, except they had to deal with a certain nuisance none of them liked. Oroku Karai—one of New York's deadliest assassins. Or mercenaries. It didn't make a difference to Raph whatsoever. None of the turtles liked the girl because somehow they always ended up having to clean up her messes. She was proven dangerous, like a snake in a vat of defenseless rats. She was also one of the most annoying people Raphael had ever met, even Mikey couldn't handle the girl. The girl just always acted like a child—a dangerously armed child.
Sighing, Raph curled his hand around the knob and turned, "Yo, Leo?" he said as he stepped into his younger brother's room, "Gear up, we gotta—"
Raphael stopped dead in his tracks.
Because before him, sat his second youngest brother, Leonardo, and the famous—most annoying—dangerously armed child. Oroku Karai. And they were on Leo's bed. Kissing. On the lips.
The two teens jolted and were abruptly pulled apart as their heads snapped in the red banded turtles' direction. They jumped away from one another and her serpentine inspired mask was off. She had a pretty face, almost-shaped eyes black as her hair, styled in a sharp, green streaked mohawk. She stared blankly at him while Leo's face changed from surprise to embarrassment as his cheeks flushed a vibrant red, possibly redder from the red marks over his eyes, before they narrowed in self-righteous anger.
The silence was thick yet awkward as Raph just stared at the two, an 'o' forming on his mouth. Karai's eyebrows disappeared into her window's peak as she looked back and forth between the two brothers. Leonardo was the first to break the silence, scowling embarrassedly at his older brother. "Do you people not know how to knock yet?!" he snapped, hands curled into fists as the back of his neck warmed.
Raph hadn't realized that his hands tightened into two shaking fists, nor did he suddenly realize that he ominously stalked towards the girl and towered over the two. A hand blindly reached for Karai's throat as he slammed the girl into the wall, the paint on the wall chipping off as dust caked the air from the force. Petite and slender hands pawed at him as they desperately attempted to pry his stronger hands off of her throat. Raph barely saw anything other than red until his younger brother leapt onto his shell, tugging the larger turtle backwards.
"Raph, let go of her!" Leo demanded as his hands fruitlessly but furiously tried to peel Raph's fingers around the girl's neck. "She didn't do anything wrong!"
Karai was making choking noises at the back of her throat as she rasped, brows furrowed together as the color began to drain from her face. Raph wasn't even aware of Leo calling for his other two younger brothers until he faintly heard Donatello's voice. He felt a string rapidly wrap around his abdomen as Michelangelo pleaded at him, but the pleas coming from his brothers cam deaf to his ears.
Finally, he felt himself being tugged away by the thin string, and three pair of arms tug him away from the girl, who dropped onto the floor with a grunt. He and his brothers toppled over each other in a mess of limbs as Raph's eyes watched the girl in front of them. Karai's hands flew up to her neck as she rubbed it soothingly, coughing raggedly as she shakily got up on her own two feet. Leo finally managed to untangle himself from his brothers as he made his way toward the girl, until Raph's hand grabbed the back of his shell and Raph towered over the girl, ignoring Leo's yells.
"Get out," he commanded and Karai recoiled as her eyes widened, her brows shooting up as she grabbed her helmet and fled from the room, Donnie and Mikey stepping aside as they watched her leave with surprise. Their heads whirled to Raph and Leo and a question hung above their heads. Raph only glared as he turned to rip his brother a new one for acting so stupid and aloof, but before he could scold his little brother, Leonardo stopped him with a livid glare, a fire raging behind his eyes. "You idiot!" he shouted, hands tightened into fists, a furious shade of red on his face, "Who the heck do you think you are? Barging into my room and bullying my date!" his little brother ranted.
Raph scowled down at his brother, his temper flaring as he stood and listened to his younger brother insult him, the typical routine between them. "Hey!" he barked, pointing an accusatory finger at his second youngest brother. "I'm your leader and your big brother. And you know we don't associate ourselves with the likes of criminals like her," he declared, "And I'm not going to have my little brother taken advantaged of by some wannabe kun—"
"Taken advantaged of?!" Leo argued, yelling over Raph as the other two in the room watched with wide eyes, and Donnie's hand smoothly clasped his high-tech Bo, eyes wary as he watched the scene in front of him and Mikey. It was a safety precaution in case things got a bit...violent. "It was one kiss!"
"You shouldn't be lettin' her kissing you!" Raph growled.
Leo gritted his teeth, eyes narrowed into furious slits. "Why? Because she's an 'assassin'?" he rebuked, putting up air quotes when he said 'assassin'. "'Cause you don't like her and that automatically means I'm not allowed to like her? 'Cause she's not a mutant?" he listed, "What freaking, frackin' reason, Raphie?"
Raph recoiled slightly at the stupid nickname; it was something Leonardo and Michelangelo had came up with when they were younger, and it was certainly one Raph didn't like at all. Raph's eye twitched, "I don't need a reason," Raph restrained the urge to strangle the little brat, "I don't want you being anywhere near her anymore."
Mikey winced and Donnie slapped his forehead. "Oh brother," he said, pinching the skin between his eyes. This was not going to end well.
Leonardo screwed up his face, tightening his jaw as he crossed his arms over his chest smoothly, "You're not Sensei," he coolly retorted, eyes filled to the brim with anger, "If I wanna see her again then I will, and it's none of your business. I'm not a little kid! I don't need your permission!" Leo shoved past the large turtle and stormed out of his room, Donnie and Mikey's eyes sliding past him, and Leo loudly slammed the door shut behind him, the other two in the room wincing at the sharp and sudden noise, the force strong enough that it rattled some of Leo's items in his room, such as the action figures from his favorite T.V. show.
Raph turned around and Mikey glared at him, throwing his hands up in the air. "Nice going, Raph!" he retorted heatedly before swinging the door open and shutting it shut as well, though not as much force as Leo. Raph clenched his jaw tightly as Donnie folded his arms across his chest, glaring at the eldest brother in disapproval, "You could've killed her, y'know," he stated factually, and Raph glared at immediate younger brother.
"She was kissin' our little brother," he snarled.
Donnie only shook his head at Raph, expression cold as stone. "That doesn't mean you should choke her to death," he argued, "Sensei would've been disappointed in you if he saw that."
With those last words, he left the room, shutting the door softly, a stark contrast to how Mikey and Leo left the bedroom.
Raph fumed silently in his head, indescribable anger coursing through his system. He forgot about patrol, training, even thinking about what type of punishment he would receive once Leo tattled on him to Splinter. His strong dislike for Oroku Karai increased significantly to the point of near brimming hatred. His breaths came out as angry puffs as the scene he had witnessed few minutes ago stewed over in his head, replaying like a record on repeat. All he could picture right now was Karai and his little brother kissing, and all he thought about it was how easily the girl could plunge her sword into the back of his baby brother's head, or how she could literally shoot his brains out with a simple push of her gun. The girl moved onto his baby brother, luring him to...ugh, he couldn't even finish the thought!
His heart pulsed into a raging fit and as he left Leo's bedroom, stomping into the living room as he decided to play a game of pinball. He vented over his frustrations and anger as he listened to the sound of the ping of the game, contemplating whether to rip Karai's head off, or to simply torture the little brat until she begged on her knees for him to stop. He was playing for about an hour or so as Splinter walked up to him, cane in his hand and his features were calm, revealing no anger that could be hiding underneath.
"Leonardo came to me earlier. Sobbing," his father began and Raph's eyes refused to make contact with his Sensei, focusing intently on the game instead. The anger was edging out of his system and his skin burned with shame as he mentally prepared himself for the scolding he was no doubt in for. "He said that you attempted to choke Karai character you had told me about."
Raph huffed, pouting, "I have no idea what you're talking about," he lied, but he knew better. Splinter could always tell when either of them were lying to him. It was like he had some sort of detector in his brain.
Splinter frowned at his eldest, and he tilted his head, "Your brothers told me what happened, Raphael," his father said, his tone calm but authoritative. "But what I want to know is why you are upset."
Raph scowled darkly as he finally made eye contact with his father, "He's too young."
"He's fourteen, Red."
He shook his head, "That's not the point, Sensei," he snapped before his rage simmered down as he looked down apologetically. "I'm sorry," he hastily apologized, but his father simply waved him off.
"Like a river over stone, Raphael," Splinter gently reminded him.
Raph sighed heavily as he let the words come out of his mouth, something he had been holding in for the past hour. "She was taking advantage of him, Sensei," he argued but to his surprise, Splinter only shook his head, looking almost confused.
"I do not believe she did such a thing, my son," he dismissed, "And if you had not let your anger blinded you, you would have learned that it was not she who initiated the kiss. Leonardo kissed her."
It didn't make Raph feel any better. "Sensei, you don't know Oroku Karai like we do. You haven't seen the things she's done. She's killed so many people—she took lives and the girl's destructive and a lying snake," he spat. "She's only going to hurt him."
"And so will many more, my son," Splinter mused, folding his hands on top of each other as they rested on his cane, "What Leonardo lacks in maturity he strives in endurance and charisma. He'll break many hearts and others will break his—even humans one day as well. That is a part of life, Raphael. Growing up."
Raph crossed his arms stubbornly, looking away as he sniffed, pouting, "Well I don't like it." he grouched.
Splinter grinned, "Nor do I," he agreed. "Do not berate your brother for such feelings. He's not a little 'turtle tot' anymore, despite my efforts to keep him from so," he chuckled as he turned around and walked back to the dojo, before turning his head over his shoulder.
"Apologize to him, Raphael, as well as Oroku Karai. Neither of them deserved your wrath."
Raph nodded stiffly, letting the words sink, before –
"Oh, and ten flips tomorrow morning," Splinter added with a devious grin, leaving his eldest and largest son alone with his thoughts.