Okay, so first of all: A better summary, more detailed:

What happens when a new mutant saves Leonardo's life? She get's taken in by the Turtle family of Master Splinter, Leonardo, Raphael, Donatello, Michelangelo, and April. Immediatly Mikey falls head over heels for her, and Leo, for some reason, seems like he doesn't trust her. Raphael finds her a rival of equal skill, and treats her like a competitive friend. Donnie treats her like a best friend, one that understands him, but doesn't see her shooting glances at him, or smiling slightly when he speaks. Oh, Donnie. Will Donnie see her true feelings? Will Leo change his feelings for her once she settles in? And will she lose her affection for Donnie and fall for a different green hero?

P.S. The main enemies in my stories are called "droids" much like their average villians, they work for Shredder and are collecting mutation serums, mutating stuff to get in their way. You know, the average twisted stuff I seem to come up with. I'm a newbie at this, don't blame me for mistakes, though fact-checks would be nice. Perhaps Slight OCC-ness.

Disclaimer: I don't own the turtles, never will. Please R&R!

AN: Sorry if it's hard to follow along in the first chapter, you don't know her name until the end. But it shouldn't be that hard.

...

"Guys! Guys, where are you?" Leonardo yelled. He was alone, it seemed. It was dark, and he was alone on a roof-top. The droids snuck up on him, in the dark, and he couldn't hear them. Or see them. When they struck, he never saw it coming.

"Guys?" he yelled one last time, "Okay, just keep your cool Leo, everything's going to be – OW!" he gasped, for a blaster had just shot him in the stomach. Lights popped in and out of his vision. He felt more and more shots hit him, until he felt no more. Then he heard a loud, growlish-scream.

"Leave him alone, creeps!" Was it one of his brothers? Only Raph would growl, though. And… it sounded… feminine.

"I said," the voice yelled, "leave. Him. Alone!"

And then everything went black.

...

She walked down the sewer. She had been watching the turtles for a while. She had seen this battle, watched as Leo was led away from his team – his brothers – and was ambushed while the guys thought that the battle was over and that Leo had left. You see, she felt kinship to the turtles – they were mutants, like her. When she was little, she had been mutated into a humanoid cat. She had fluffy, fuzzy, golden fur. Her eyes were deep amber, and she had dark spots and stripes. She wore black yoga pants, and a dark purple tank top. Her tail was long like a cheetah's, and kept flicking back and forth as she walked.

She carried an unconscious Leo in her arms. He was a little heavy, but, she was strong. She just had to retract her claws.

She fought mainly hand-to-hand, but she had a belt full of various knives (some poisonous!) and she had inch long claws. To make them even more affective, she had made steel claws three-inches long that were razor-sharp, and she attached them to her claws when fighting. But they were attached to her belt, for now.

Leo groaned. Her tail flicked up and gently stroked the side of his face, "Shh, leader of the Ninja Turtles. You'll be home soon. But till then, you are too weak."

She then found a highly-concealed door. Her tail went up and rapped on it sharply. She heard a voice "Guys! I bet that's Leo!" "Shut up Mikey, that's not Leo! He never knocks; he knows how to get in!" "Well Raph, what if he forgot? Not everyone can be as smart as me." "Stop your arguing! Open the door Raphael. See who it is. It may be Leonardo."

Raphael opened the door. He gaped at the sight that met him. "What did you do to him!?" He lunged at her, who sidestepped him and smirked, showing her sharp cat teeth.

Master Splinter appeared in the doorway, "Raphael, show some dignity. Do you think that the creature that did this to Leonardo would bring him back, or even be able to find this place? This mutant is friendly, I believe."

Donatello appeared, "Then who are you? And what did happen to Leo?"

Then came out Michelangelo: he stopped, looked at her with wide eyes, and said, "Hey, aren't we being… what's the word? Polite! Yeah, polite! Why don't you bring your pretty self in?"

She stepped forward, into the doorway, and turned to face Mikey. She curled her tail around his chin, "You come on strong," she purred, "I like that." Then flicked his nose and walked in.

"Nice place you got here." She looked around, "Better than the random allies I live in when I get sick or hurt."

She laid Leo down on a mat and kneeled beside him. "He's badly injured. Those blasters are powerful. He could be out of it for weeks."

April had appeared, "How can you tell?"

The cat-girl stood and fixed them all with her fiery eyes, "My past fifteen years… have not been the best… I was mutated as an infant. My parents abandoned me. So I've grown… studying. I had a good friend – a scientific doctor – who I met. Someone had ambushed him in an alley, which I had currently been hiding in. I saved his life, we became friends, and he taught me everything he knew. Helped raise me for a couple years. But then one day he disappeared. Haven't seen him since. But, recently, I've found a new point of interest."

She nodded at Raphael, Donatello, and Michelangelo, "You four. I've seen you fight. I was watching tonight when your leader was pulled away. You left, so I followed him. They ambushed him, nearly killing him, so I jumped in. And, you, tough one," she scowled at Raphael, "I saved his life. You're welcome. And now I'm about to bring him back, instead of just letting him get bed rest for the next month or so."

She turned back to Leo, and took a small, blue stone from her belt. She put it in her left palm and then out came her claws. She held the stone in between her thumb claw and pointer claw. She started scraping the rock so that a powder started to sift down, and she let it fall onto Leo's face and body. Once the stone was gone, the powder started to glow.

She smiled, "There. He'll be awake in a few minutes." She went back the way she came. Mikey reached out and grabbed her wrist, "Wait, you're leaving?"

The mutated girl turned back to him, eyes solemn, "Of course. I have intruded in your home. I meant only to bring back your brother. I should go."

Just then Leo woke up, sitting up sharply, "What happened? Who was the girl? Where are the droids! And why," he said with a groan, "does everything hurt?"

She looked back and nodded at Leo, "The droids ambushed you; I rescued you, brought you back, and used a rare healing stone on you. You will still be beat up for a few hours, but not months. Farewell."

Mikey pouted, "We don't even know your name!"

She turned back to him for a second, fixed her wise eyes on him and said (with a slight purr) "My name… is Willow." And she was gone.

...

AN: Throughout the story there may be some MichelangeloXWillow DonatelloXWillow and LeonardoXWillow. I like to keep you in suspence. Willow's a crafty one! ;)