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Kimimaro was of the Kaguya clan in Naruto, a clan whose kekkai genkai allowed them to manipulate their own bones in battle. They were fierce warriors until they were defeated, and Kimimaro served Orochimaru after the fact.
Childhood (Will contain spoilers)
Child Kimimaro
Being of the Kaguya clan, Kimimaro had been born with a powerful bone manipulating kekkai genkai. His clan, even as he was younger, had been a warring nation. But because of Kimimaro's abilities he was locked up in a cage for most of his life, only released to fight, fearing that if he were left unchecked he would grow out of control. But he didn't mind, he was of use.. that was all that mattered to him. But, Kimimaro's caged existence came to a close when his clan attacked Kirigakure and were wiped out.. save, for Kimimaro.
Kimimaro, as a child had met Haku and Zabuza, asking them if they were of the mist village, a land they had once tried to slay. Zabuza said no, and left without a second thought, as he did think Kimimaro was a threat. He had been terribly wrong, but he never knew... and never would.
Kimimaro was found by Orochimaru during this time, and was raised by him after realizing that he was an orphan. It was in this time that Kimimaro grew to love Orochimaru, and, like all of Orochimaru's other servants, wanted to be his next body replacement.
Summary (Will contain spoilers)
Kimimaro Kaguya was a member of Orochimaru's Sound Five, and also the reason the entire organization ascended numerical value. Once he had been introduced to the entire organization, they quickly attacked him at the thought of having someone favored over them by their master. Kimimaro defeated them all with ease, and proved to be Orochimaru's greatest asset until being stricken with disease, and forced to remain bed ridden for his opening portion of the series.
Because of his disease he found he was no longer useful to Orochimaru, he couldn't be his beloved master's body. For once the quiet seemingly near stoic figure cracked.. he would be seen silently crying. Kimimaro lived to serve, wishing to be of use, much like his childhood, he only fought because it made him feel wanted. And now, here with Orochimaru, if he cannot do what Orochimaru wished of him, what good was he?
Mostly on force of will, Kimimaro went out into the field once again, after falling ill to help aid in Sasuke's recovery. It was a way for him to serve Orochimaru , even if it was to give him a body that wasn't his. Sasuke was important to his master, his father figure, his confidant, and he had to make sure Sasuke reached Orochimaru, even if he had to deal with -trash- ninja to get it done.
Before leaving however he payed a farewell to Jugo, a friend, a person whom 
Kimimaro and Jugo
only he could calm his killing urges, one of Orochimaru's experiments.. Though Jugo warned him from going Kimimaro insisted. Saying that Sasuke was his successor, and he needed to make sure he arrived. He thanked Jugo for their friendship and the two parted ways.
Kimimaro managed to easily secure Sasuke's barrel, but he wasn't without a set of 'trash' ninja hot on his heels, one being Naruto Uzumaki Which Kimimaro dealt with easily. So easily infact he bought plenty of time for Sasuke to finish his transformation and escape on his own. Done toying with the 'trash' ninja, Kimimaro was about ready to finish the orange ninja off when lo and behold newly healed Rock Lee came to his aid, seconds before Naruto would be taken out.
With the torch passed, Rock Lee took over the fight and Naruto escaped after Sasuke. Kimimaro was more than a match for the ninja, but once Lee went into his 'Drunken Fist' technique did Kimimaro became taken off guard, Lee's movement's were random and he was unable to predict them. Once Lee sobered up, however, Kimimaro once again had the upper hand, and was about to kill Lee when Gaara appeared on the scene.
Though Gaara put up a good fight, he was unable to do anything but hold back 
Second stage.
Kimimaro, and once Kimimaro used his second Curse state, he destoryied all resistance Gaara gave and made his advance. But, once Gaara insulted Kimimarow's comitment of Orochimaru, the male lost control, like no one had ever seen. Though the sand ninja used his ultimate attack, and turned the whole fighting area into sand, trying to crush Kimimaro underneath while Gaara and Lee remained above, the enraged Ninja returned. Kimimaro used one last technique, the sand impaled by his bones jutting up like miles of deadly ivory spikes as he came at the weakened Gaara and Lee with everything he had..but...
He stopped..

Moment's from the end.
Seemingly frozen in time, mere millimeters from impaling the two ninja..he just..stopped. His body had quit.. Kimimaro had died in the middle of his attack. The two Ninja lucking out moment's from disaster.
It became clear, though Kimimaro served a evil person, such as Orochimaru, he was loyal, and from even allowing Lee to take his 'medicine' during their fight, he was honorable. Kimimaro was never an evil person, he was a person who wished for one thing and one thing only. To be useful.