Well, basically, I will make legendaries a little bit stronger than normal final-evolution Pokemon. That way, they can be beat. Also, nobles are the only ones that can have legendaries. I made them a trademark of people from noble houses. For instance, if someone sees a man or woman walking down the street with a Xerneas following them, they can easily say "oh wow, they must be someone extremely important". It's to prevent overflow of legendaries, ya know?