First you must buy every other character there is to buy (except Anbu Kakashi)
Then you must complete at least 20%+ of everyone's missions, and 40%+ of Kakashi's Missions
You must beat Kakashi's S Rank Mission Vs. Gai then do it again
But this time it is against Anbu Kakashi, you will have to beat him.
Once you beat him he will be unlocked in the shop for 100,000 ryo