The Rule Follower—
Rose loves rules. They're set up so nicely and you know exactly what to do and what not to do. You know what is expected of you and you know exactly what to do to ensure that you get the expected result.
But she can't help but wonder what the people who break the rules all the time feel like. Roxie and James seem to always be having fun and she finds herself envious of her classmates who seem to so easily say "Screw the consequences!" because Rose just can't seem to find a reason to screw the consequences.
If she finds a reason to, she will.
When she finds out Louis' affinity for art, after a long fascination with tattoos, she begs him ink her. And that's how she got the little star tattoo behind her ear, covered by her hair. She smirks to herself; her classmates don't know just how much of a rebel she is.
And then she re-meets him. Scorpius Malfoy, rebel extraordinaire. He has a tattoo too, one of a scorpion on his back. She has always found him fascinating and suddenly she finds herself dancing with him suggestively at a party, grinding her hips into him as she breaks all her rules.
She surprised him, she knew, by shagging him that night. He was even more surprised to find her tattoo, demanding to know just when she got it. She delighted in the fact that she had been inked before he had.
So she roamed the halls of Hogwarts happily, giving off the impression to everyone else that she is the epitome of a girl with a stick up her rear end, but knowing that the one person who really mattered knew her true colours.
Because Rose Weasley was a rebel at heart, you just had to dig beneath her carefully constructed façade before you found out.