Kimimaro was one of the original Sound Five and appears near the end of their journey to protect Sasuke so he can reach the second level of the Cursed Seal. Kimimaro is the sole survivor of the Kaguya clan, the clan who can manipulate their bodies skeleton frame. He is well suited for close combat fighting and distant fighting. Since he can regenerate and make new bones, he has a endless supply of bones he can use against his opponent.
Kimimaro suffers from a illness which was not thoroughly explained through out the manga. Taking a look at the facts from the manga, it could be considered a cancer-like disease since it effects his whole body and makes him sick. While he does suffer from an illness he still is strong enough to hold his own against most of the opponents which approach to fight him. In this case that would be Gaara and Rock Lee.
Kimimaro also knows the 5 special dances which he can use at his own will to kill his opponents. All of these dances can only be used by him and any other Kaguya clan members. His attacks are all devastating and can also be used for defense. He is well taught in the way of the sword and uses his own bones as weapons. He can form his bones into any shape which makes him a fearsome ninja in the series.
Kimimaro could be considered like another Haku. He has a deep gratitude and love for Orochimaru and he is willing to do whatever it takes to help Orochimaru complete his plan of destroying the Hidden Leaf Village. We don't see him in the attack on Konoha because he is very ill at the time but it has been said that if he were in top condition the whole plan would of went as Orochimaru had planned and the village would have been demolished. He is a powerful ninja but his illness is his weakness. He died due to his illness just before he could land the finishing blow on Gaara, who was out of chakra.