Mia is a young woman, not quite out of high school yet. She's a sweet-looking teen who dressed up modestly with a minimum of makeup, and she'd look like any other girl. There's no indication that in truth she's a seductive vixen with an appetite for sex and kink that far outweighs her age, or experience. Take the most shameless slut you've seen and plant her in the body of a young woman who shouldn't know so much. That's Mia.
The story is one of seduction. She knows what she wants and is fearless in getting it. You play the older man, in this case her sister's boyfriend or husband. There is no way you should even be looking at this girl, but under her constant and expert advances eventually you will give in. The sex is taboo, illicit and oh so good.
The story is one of depravity. When you give in, you give in all the way. Finding her endless willingness to do anything, you will meet her step by step. This is going to be a story of full-out, no-limits sex. When Mia convinces your character not to treat her as a girl, you will take it seriously. Mia has a passion, and a lack of limits, which is startling to see. She wants you to murder her vagina. She takes the most brutal facefucking with a thrilled laugh. She cums buckets as you bind her mouth with duct tape and fist her. She doesn't just want to go down on you while her sister is in the other room, she wants to rim you. She is like no woman you've ever met, and the contrast of her young seductive smile and her capacity for perversion is an intoxicant you can't believe.
The tone of this story is very rough, very intense, very nasty. And yet the tone is one of joyful excitement, not submission or force. Mia wants to find out all the ways there are to be used, but she drives the action as much as the other characters. She is a sexual tornado who drives your character to do anything to her and he does. I would expect many kinks in this and probably someone willing, at times, to play other or multiple characters. Tell me how you would match Mia's endless desires and maybe you'll get to play with her.