She was a precocious jounin, one of the many shinobi lost in the battle against the Kyuubi. She was also one of the rarer, more determined ninja. Dame Fortune and Mother Nature had blessed her with intelligence, skills, and looks (or so all around her thought, but she was oblivious to it). They could only watch as the demon fox killed the object of their blessings.
Severely wounded, she staggered up from where one of the Kyuubi's lashing tails had thrown her and coughed, "Tajuu K-Kage Bunshin no J-j-Jutsu!", barely managing to hack out the last word. "D-dang it...." The army of shadow clones, though sizable, contained copies as severely wounded as the original.
Growling in disapproval, she nevertheless launched her clones into battle. A few demonic minutes later, she cried out in pain: a huge orange paw had swept her aside, and her body slammed into the splintered remains of an old broken tree. She gazed blearily at the dark sky, wishing that she could at least see the stars (the fires scattered and rose under the Kyuubi's watch, obscuring the light of the stars), unable to move: the crash had snapped her spine like a dry twig.
She faintly heard the Hokage shout a few words and imagined him springing at the demon—but that was impossible, because the sealing jutsu needed so much more than a complex series of hand formations.
A second later, a familiar, familiarly masked silver-haired boy dropped to the ground beside her, trying to pry her off the splintering tree, wailing, "No! No! Yukiko-oneesan, don't go, Mom—Mom and Dad have gone already, I can't live alone!"
She smiled feebly, then choked out several drops of blood. "'Kaka-chan, take care of yourself. Grow up to be a jounin like me, 'kay? Though I guess I'm not so great to die right now, but when we next see each other, I hope you'll be a jounin, and maybe even get that Sharingan eye you want, neh?" She reached up, one finger touching his left eye. "Good luck, 'Kaka-chan."
"No! You won't die! I won't let you die—no—YUKIKO!"
"Shh," she whispered tenderly, her glazing eyes calmly watching another silver-haired boy standing beside Kakashi, drawing a katana that was strapped over his back. Her eyes met his: they were like two chips of blue-green ice, cold and reserved. The unknown boy placed the base of his sword against her forehead, and a few seconds after he had lifted it away, Hatake Kakashi gazed desolately at his big sister's pallid, cold face and broke out howling at the moon.