Kimimaro

is the fifth member of the Sound Four, thus temporarily changing their group name to the Sound Five. The original four were forced to accept him against their will, as Kimimaro's combat abilities far exceeded all of their skills combined.

Background
Kimimaro was the sole survivor of the Kaguya clan, who were battle loving barbarians, however once in a while a Kaguya was born with the ability to manipulate his bones that one being Kimimaro. The Kaguya clan was wiped out when they attacked the Hidden Mist Village (seemingly for no reason other than the desire to kill). Having underestimated their opponent's defenses, they were slaughtered to the last man, with only Kimimaro surviving.

Kimimaro was referred to by Orochimaru as the most powerful member of the clan, having "abilities that even that war loving clan feared". Because of this, he was locked away in a cage for most of his life, only released occasionally when needed to fight other clans. He retreated from the fight with the Hidden Mist Village, which is probably why he survived. Afterwards he was found by Orochimaru and recruited as one of his most loyal servants.

While belonging to a clan that loves killing, murder, and slaughter, and being the most powerful of this clan, Kimimaro was not truly violent because he never really knew the outside world. Thus, the only reason he fought was because his clan needed him to do so, and being needed by someone was the one thing he truly desired (until he meets Orochimaru, where his desires change to serving him and ensuring the continuation of his master's dreams and ambitions).

Personality
He had a strong devotion towards Orochimaru, bordering on religious zealotry or considering him a father figure, as Orochimaru was the only person to care for and inspire him. On his way to the Hidden Mist, he encountered Zabuza Momochi and Haku, apparently shortly after they met each other, since Haku is still young. Their relationship parallels his relationship with Orochimaru in many ways, as both had similar backgrounds and found a purpose in living for another.

Kimimaro demonstrates extreme distaste for "trash" ninja, as he puts it. He generally attempts to "clean it up", by killing the person who is "trash", even if they are an ally (he plotted to kill Tayuya after she defeated Shikamaru because she was unable to deliver Sasuke by sunset).

Death of the Kazekage
It is revealed in a flashback that Kimimaro was at the site of the death of the Fourth Kazekage.

Sasuke Retrieval
Kimimaro died when he attempted to bring Sasuke to Orochimaru as a way to end his life and thank Orochimaru for his doing. He succedded in taking the barrel Sasuke was in, but Naruto Uzumaki caught up with him to take Sasuke back. Kimimaro easily held off Naruto until Sasuke could get away on his own. Rock Lee (after recently having surgery), saved Naruto. He easily defeated Rock Lee, even after Rock Lee attempted the Drunken Fist style of taijutsu (which is the ability for ninja to power up by drinking alcohol), but Gaara came and saved him, who appeared as a result of renewed relations between Sunagakure and Konohagakure. Though he was able to hold off all of Gaara's attacks, and even gained the upper hand near the end, he succumbed to his illness just before he could kill Gaara with a surprise attack, and died. Gaara came to the conclusion after the fight that no one can defeat loneliness, even if it means to care for an evil person like Orochimaru. Gaara comments that Kimimaro had the same eyes as Sasuke, which say (so to speak) "I want to confirm the value of my existence."

It is later revealed that he was with Orochimaru when he managed to trick some of the Fuma clan into joining with him sometime prior to his invasion upon Konoha.

Part II
Kimimaro was mentioned by Jugo of Team Snake when he was fighting Sasuke, as Jugo claimed Sasuke and Kimimaro were the strongest people he ever battled. Kimimaro was also capable of controlling Jugo's berserk rages without having to battle him, which made Jugo care for Kimimaro as a friend (and the reverse was true as well, since Kimimaro thanked Jugo since it was mainly his abilities that made Kimimaro's so great). Jugo's reasons for joining Team Snake include the fact that, after hearing that Kimimaro had died to ensure Sasuke reaching Orochimaru, he believed that Kimimaro's spirit continued to live on in Sasuke, despite how one major difference between Sasuke and Kimimaro was that Sasuke had no care for Orochimaru, and only went to him for more power.

Abilities
Kimimaro's abilities were incredible, as he was said to be as powerful as Orochimaru himself. His abilitites made him well suited for close combat fighting, though he could use long-distance combat as well. His abilities come from his bloodline limit Shikotsumyaku, which allowed the user to use their bones to fight. Specifically, it allowed Kimimaro to combine his chakra with calcium, making his bones flexible as well as strong. Despite being able to use these bones normally, Kimimaro could put them in the forms of taijutsu attacks (which he called dances), to give him a variety of abilities.

Like many others of Orochimaru's closest followers, Kimimaro was branded with a curse seal, which gives him greater power at the expense of his free will. He bears the Cursed Seal of Earth, which is said to be equal in power to the Heaven seal branded on Sasuke and Anko. When in his Level 2 form, his appearance is similar to that of a dinosaur, with dark gray skin, six large bone spines on the back of his chest, and a long bone spiked tail. He also appears to have much greater control over his seal than any other character seen so far since he was able to restrict its spread to his chest in order to grant him a greater surge of power without activating it fully. Despite his incredible power, Kimimaro eventually lost it and became bedridden due to an unknown disease.