Well, there seems to be a simple answer.
That they move with surprising stealth, sneaking up on you when you don't expect it? What about their wardrobe? Do they have viper fast reflexes? But yours sound far too good at it. Like they've been practicing. Child Black Ninja Master Costume Do you find excessive dark pants, shirts, and masks? Hm. After all, it could be useful to have a stealth master at some point.