Each personality in the ego is a complete separate person, with its own likes, desires, and plans. If you observe your actions and behaviors you will find many of them to be incompatible. For example if you are trying to lose weight, sometimes you will have willpower and eat those veggies, sometimes you will refuse to restrict your food intake and will eat that cake. If you then remember your mental state when you took those incompatible actions, you will find that you were two different people when you made those decisions. The person that ate the veggies is not the same person who ate the cake.
So this is your ego definition, a collection of personalities each taking control for a few moments with the end result being we go nowhere in our lives and end up frustrated and depressed. There is hope, though, for those with an aim. If your aim is enlightenment or spiritual development, then you will find some of those personalities agreeable to your task, some neutral to it, and some hostile.