Steal is an ability that you can learn from Rikku’s section of the Sphere Grid. While anyone can learn it to use in battle, this trophy/achievement requires you to have Rikku use the skill. It requires you to successfully steal from enemies 200 times, which might seem like a lot, but you should always be stealing with Rikku. Note that you are guaranteed a steal on the first attempt, then the chance will be cut in half on each attempt after the first successful one.