Mandy Carson is the scarily devoted girlfriend of Billy Grimm, the one who declares, in front of Beckett and Castle and to Billy, that the "only way you're going back to jail is over my dead body". She was afraid that Billy would get himself into jail again, if he went after Derek Brookner, so she told Derek that if he wanted no troubles he give Billy money. Derek did as she said, and came with $25000 as an apology. When Billy found out, he wasn't angry that she went behind his back, but instead kissed her, because he was touched by it, since he thought it was 'the most romantic thing he ever heard'.