How does he react? Does he seem consistent with his character from our earlier view of him? How does it make him vulnerable to the King?

The sight of his insane sister deeply grieves Laertes. Ophelia handles all those present gifts of flowers, each symbolizing a reproach to the receiver. She sings another song about her dead father and exits abruptly. As she leaves Claudius tells Laertes to inquire into the matter as deeply as he wishes, confident that he will find himself aligned with Claudius against Hamlet. Laertes agrees. Laertes wants revenge for the death of his father and the condition of his sister, this makes him vulnerable to the King's manipulations.