Who are Hamlet's foils, and in what ways do their characters shed light on his?A foil is a character who is like the protagonist in some respects...

One of the foils to Hamlet is Laertes, they are similar in age and situation though of course Hamlet has more of a direct path to the throne, etc.  Laertes is also interested in being off at college, he really cares for Ophelia as does Hamlet, and he is an accomplished swordsman.

But Laertes is also a man of action, he responds immediately to situations.  He leaps into Ophelia's grave, he is desperate to find a way to avenge his father's death and Claudius actually has to slow him down and get him to consider things a bit more.  On the other hand, Hamlet is always considering everything at incredible length rather than just acting.  Laertes serves to bring out this incredible inaction on Hamlet's part.