Years ago, when Sasuke discovered his brother's horrific doings, he spent the night in the hospital reovering from the effects of the Mangekyou Sharingan. At one point, he was sitting on the pier where he had learned the Katon technique. Diving into the water, he made a terrible vow of vengeance.
Sasuke resurfaced and started, surprised to see a girl of about fifteen or so standing in front of him. Her eyes were the color of jet and her hair, unbound and feely flowing, matched their hue. She was gazing at Sasuke with a solid determination, but behind that there was an infinitely sad expression.
Who are you? Sasuke demanded.
The girl turned away and said nothing.
Sasuke grew impatient. What are you doing here? You're trespassing...a lot of people just died here! What do you- But he stopped short.
A small tear had formed in the girl's eye and traced down her cheek. Turning her head back she took a breath and shuddered.
Sasuke...don't you recognize anything about me at all?
The boy was angry now. I have no idea who you are! Tell me now or leave!
Kari knelt down and placed her hands gently on his shoulders. Somehow, Sasuke was disinclined to push them off, but only gazed intently into those eyes...
Those eyes! His eyes! They were...Uchiha eyes...like his mother's...
Like Itachi's!
The boy was shaking now. Please...who are you? he gasped.
Kari stood up again and said quietly, My name is Kari and until now, I was never allowed to speak to you. But seeing as Itachi is gone...there is no reason to hide myself from you anymore. She shuddered again. I know that many have died, Sasuke. They were my parents too. My mother. My father. The only one I was able to save was you by convincing my twin brother that you could be useful to him...
Sasuke bolted upright. You're twin...brother? But Itachi didn't have a twin. He was my older brother. I never had a sister...
None that you were aware of to be sure. I was taken away from your family when I was about five. Itachi and I were twins. We were separated because the clan feared our power combined. We were both well-advanced for our ages and that concerned them. I was never allowed to see you. But I was always near you, Sasuke, protecting you from afar. Last night was my hardest task...convincing Itachi not to kill you.
Sasuke stared. It couldn't be. It was impossible. He had a sister...and Itachi had a twin. But he suddenly reflected on a memory. His mother had been looking at a picture of a young girl that looked a great deal like Itachi. She had been crying. When he inquired about it, his mom grew angry and told him never to speak about it again. The girl was astoundingly similar to the one standing in front of him.
Were you the reason Itachi was always so angry?
Kari nodded. It was hard on both of us. But while I eventually forgave the clan heads, Itachi never could. He grew wrathful and rebellious. Learning about the Mangekyou made things worse. Kari sat on the edge of the pier and was joined by Sasuke. Last night, I didn't catch wind of what he was doing until he had already killed our parents. I got to him a few minutes before you arrived. I cried and asked him why he had done it. He said it was to bring us back together and told me to come with him. I refused and then I asked him what he would do with you. He simply stated he would kill you like the rest. I begged him not to, then told him that you could be just as powerful someday and that would be useful to him. Of course, I had no idea, I simply wanted him to let you live. He considered for a moment and then said he would spare you only if I left the two of you alone. I did...for the most part. I watched everything from the edge of the farthest house and had an attack ready in case he should try anything. When he sent you to a dellusionary state and continued to torture you, I jumped in. I quickly stopped the dellusions with a technique I had developed and unleashed my attack. Itachi left...more from boredom than from being afraid of me. But he had left and you were safe.
Kari gazed at Sasuke and shook her head. I wanted to follow him and finish it off. But I was afraid for your safety. I decided that you were worth more than he was.
Sasuke was crying at this point. He let his head slip into Kari's lap and Kari stroked his hair gently. I'm going to protect you from now on Sasuke...
Kari entered Konoha and headed for the Hokage's mansion. Yes, I swore to protect you. But now you've been poisoned by Orochimaru. You've allowed him to take over your body and thoughts. Now, you will force me to go after you...and drag you back against your will. And if that is the cost, then so be it.