Kimimaro Kaguya, the fifth member of the Sound Four, is one of the many antagonists in the Naruto series who is subservient to Orochimaru. The sole survivor of the 'fiere and battle loving' Kaguya clan, Kimimaro has an exceptional bloodline limit that allows him to manipulate his skeletal structure to his volition during battle, and makes his bones stronger than steel, which he openly admits when fighting Rock Lee and Gaara.
As mentioned earlier, Kimimaro was the only survivor of the Kaguya clan. The clan had been wiped out after underestimating their opponents in Kirigakure, leaving Kimimaro the only survivor, who was later found by Orochimaru and taken under his wing. Because of this Kimimaro became very loyal to the exiled Sannin, and he quickly became a favorite and one of the strongest fighters that Orochimaru had on his side because of his background. Though, the Kaguya boy was not as violent as his relatives as he had not stepped proper foot into the outside world.
After earning enough of Orochimaru's trust and being accepted as the strongest, Orochimaru planned on using Kimimaro as his newest host or body vessel for himself, but quickly threw the Kaguya aside after he was discovered to have an unknown disease, thus deeming him useless in Orochimaru's eyes. Kimimaro worked with the Sound Four for a while afterwards (who renamed themselves the Sound Five because of the extra member), dressing in similar attire as they did. The original members of the Sound Four were not pleased with this however, only accepting him into the group because he was stronger than all of them.
Though it was thought that Kimimaro had no real friends, he was very close to Jugo, who was one of Orochimaru's test subjects for various things including the cursed seal. Kimimaro thus became the only person who could surpress Jugo's killing instincts when the cursed seal activated.
 Chunnin Exam Arc
Kimimaro made an appearance in the Chunnin exam arc, being the person who killed a Genin named Shiore so Orochimaru could impersonate him, and also being one of the reasons for the Fourth Kazekage's death, and the death of his four bodyguards. During this time it is also revealed that Kimimaro had a debilitating illness, and was forced to go under intense medical care under the watchful eye of Kabuto Yakushi.
 Sasuke Retrieval Arc
Kimimaro's second and final appearance happened in the Sasuke Retrieval Arc. Despite the protests from Kabuto, Kimimaro left the medical care that Kabuto provided for him to bring back Orochimaru's latest choice in a vessel, Sasuke Uchiha, because he had deemed himself worthless after learning about his disease and how Orochimaru had no more use for him. He meets up with Sasuke and they begin to fight, only to be interrupted by Naruto who fought against Kimimaro and told Sasuke to run. Sasuke heeded his words and left. Seconds before being killed by Kimimaro, Naruto gets rescued by Rock Lee who sends Naruto after Sasuke. Rock Lee and Kimimaro begin to fight.
Kimimaro, being extremely strong, was easily able to hold off Lee and deal many blows to him. When Lee used the Drunken Fist Style however, did Kimimaro struggle a bit as he was unable to do anything. Lee left the Drunken Fist style stage and then Kimimaruo came close to killing Lee, who was then rescued by Gaara. Gaara and Kimimaro face off, and an intense battle ensues. Kimimaro uses his kekkei genkai to retrieve weapons made from his bones including a whip made of his spine, and Gaara uses a move which turns the entire battle field into a sandy pit. Now that Gaara is out of chakra, Kimimaro enters the second cursed seal stage and he turns the battle field into a forest of bones. Right as he prepares to deal the final blow to the helpless Gaara, Kimimaro's illness takes over and the last Kaguya clan survivor is no more.
Coming from the Kaguya clan, Kimimaro is an extremely strong fighter, who was the strongest out of his entire clan, thus why they locked him up when he was younger, only to let him out whenever they needed him to fight with them. In doing so, Kimimaro was able to hone his skills quite well, and master control over his kekkei genkai, which allows him to manipulate his bones and use them as weapons. An example of this is the whip that he made out of his spinal cord.
Along with the extremely powerful fighting skills and the kekkei genkai, Kimimaro also received one of Orochimaru's cursed seals, and when he gets into the second stage of the seal, his skin turns a dark brown color and he resembles a type of monster. The bones of his will come out more profoundly and his chakra and bone mass is enhanced.
Kimimaro's main weakness lies in his stamina though, as his disease prevents him from overworking himself too much.