Kabuto first appeared in the chuunin exam as a leaf student who was extremely weak but extremely smart. Though it was soon short lived when he was shown as a hidden spy of the Sound village. Kabuto is not weak but extremely strong and is able to hold his own to Kakashi and some of the best ninjas.
Kabuto is Orochimaru's right hand man and does only the most difficult missions for the sound because he is the only one Orochimaru can slightly trust. Kabuto rarely questions anything ordered by Orochimaru and follows up every command.
Kabuto was found by a medical team by the leaf at a young age and was not really good at any particular genre of fighting but as shown in the series he is extremely talented in tactics and medical jutsu. So much so that he was able to deal with Tsunade while his body movement was reverted.
Kabuto is a very strong ninja for the sound and is the second in command of the operations that happen in the Sound village. He takes care of all the arrangements for Orochimaru while his arms were unable to move. Kabuto is a extremely strong ninja but some have rumored that he is part of the Akatsuki.