Cressida was there because she wants to get to know Greg better. She brought a small trophy and a gold star. She found the trophy in smuggled, and she charmed the plaque so that it had his name under an award for being badass. The gold star, because she says she owes him one (I forget the reasons), she puts on his shirt where it will stay, with magic, till the end of the night. After the party, the star will flutter off his shirt and turn into a cat toy. She's showing off a bit...sorry.