The character who interacts with Percy after he cuts off the head of the Fury in the book is Luke Castellan, the head counselor of Hermes' cabin at Camp Half-Blood.
Luke says, "I thought you'd be smarter than that, Percy. You've just messed with the Furies. You think they're going to let you get away with this?"
If you're thinking of a different scene or book, please provide more details so I can assist you further!