What is the importance of Young Fortinbras in the play?

Young Fortinbras hates Denmark. His father, the King of Norway, was killed in the wars with Denmark. Young Fortinbras is a hot head of "Of unimprovèd mettle hot and full". He has assembled a small army of thugs on the outskirts of Denmark. He means to take back by "strong hand" the lands that his father lost. After Hamlet Sr. dies Fortinbras sends (scene 2) and ultimatum to Claudius. Young Fortinbras works as a character foil to Hamlet. Young Fortinbras is a man of action as opposed to Hamlets meditative over-thinking.