Fortinbras is a fascinating character in this play. Although he only actually appears in the final act, his presence lurks throughout every act, as Shakespeare uses him as a foil to contrast against Hamlet. The parallels between them are clear: both have lost their father to an enemy and both are prevented from becoming king by an uncle who has taken the crown instead. However, what is far more important are the differences between them. Fortinbras actively does everything he can to get vengeance against Denmark and Old King Hamlet (even though he is now dead) by regaining the rightful lands that were taken from his father. Hamlet, on the other hand, spends most of his time procrastinating and deciding what to do and how to do it when it comes to his vengeance.
Fortinbras is mentioned again in Act II scene 2 by Valtemant, the ambassador of Claudius to Norway, when he reports that the planned attack against Denmark first mentioned in Act I scene 2 has been put a stop to. The old king of Norway stopped his nephew from pursuing his posturings of war:
...sends out arrests
On Fortinbras, which he, in brief, obeys,
Receives rebuke from Norway, and, in fine,
Makes vow before his uncle never more
To give th'essay of arms against your majesty...
Fortinbras is mentioned in this Act as a reminder of what he has already done to try and gain vengeance. Even though it appears in this scene his attempts have come to a halt, he has already done so much more than Hamlet has done, even though both are in a similar position.