Because she steal Harry his car and then crashed it, leading to Harry getting arrested next week. There were a lot more scenes of her, but I'm sticking to the one that show the motivation for stealing the car and what happens during and if relevant a bit afterwards to round it up again.
Doug, Danny, Amy, Tegan - The club of people turning in their grave because of what Ste's turned into.
Why is Mercedes important for the storyline?
Because she steal Harry his car and then crashed it, leading to Harry getting arrested next week. There were a lot more scenes of her, but I'm sticking to the one that show the motivation for stealing the car and what happens during and if relevant a bit afterwards to round it up again.