Not every kid wants to be the white knight in shining armor. Not ever boy wants to fight dragons. Or save the princess. Some kids want to be the dragon. They want to breath fire. They want to be the undead thing the princess needs to be saved from.
Your son is particularly into ninjas. Not just any ninjas though, specifically evil ninjas. He just thinks they are the coolest things to ever hide in the shadows. He tries to hide behind the couch so he can jump out at the dog. The dog always sees him coming though, that’s when he starts in on the “I need to look like a ninja in order to be able to sneak up on Max.” You don’t tell him that Max could still smell him coming even if he couldn’t “see” him.
Max aside you think it might be fun to watch him “sneak” around the house in this Kids Skull Ninja Costume. And sure he might say he is working for a dark master, trying to achieve dark goals. The “dark goals” being getting to the cookie jar without you seeing, and trying to give Max an ice-cream cone because it is funny how he eats it. He will put on this costume, and ask to be friends with dragons, and to help the dragon fight the knight in shining armor. It will be so cute to watch your little evil ninja.