Aoi is the diabolical ninja who was hired by the Wagarashi clan which is the rival of the Wasabi clan. His job is to stop Idate, a runner for the Wasabi clan, from winning the Todori Burning Fire Race. The story behind Aoi is much deeper though as he was a previously a Konoha jounin ninja.
Aoi betrayed Konoha by having his student Idate steal 2 special treasures from the village. He lied and promised Idate that he will become a chuunin. These treasures were the second Raijin Sword and a special scroll of special jutsus. After this event Aoi went back to the Hidden Rain Village, but was caught up by Idate's brother Morino Ibiki, who then was caught by the Hiden Rain and tortured to translate the scroll. After this incident Aoi has been with the Rain Village ever since and does even the most worthless jobs that ninjas would do.
All of his skills involve his umbrella and his needles but this is to be expected since he is now a rain ninja, though what sets him apart from all the ninjas is his use of the special sword that is Raijin, which he uses constantly when he lacks any other real power to inflict damage on his opponents. His skills seem pretty basic even for a jounin but he still manages to give most ninjas a pretty hard time in a fight.