Sheik like Smash 4 is her own separate character after sharing a spot with Princess Zelda, whom she really is under all that Hylian-Ninja gear.
She's much faster than being just Princess Zelda, as she has been since Melee. Not many characters are as fast or quick as her.
While most of her attacks lack power, she does have some useful tricks in her arsenal.
Her final smash is the Sheikah Dance which replaced the Light Arrow from previous games. She user her knife and anyone caught by it will be in a world of pain. I didn't think it was that useful.
When Kirby takes the power of Sheik, he use needle storm which are just a bunch of little daggers that can be thrown. It isn't too useful.
Online I didn't do so well with Sheik, only winning just a few rounds. I just think she's under-powered and her fast feet don't make up for it. While I still like her, she was far far from my favorite in the game.