How is Horatio a foil to Hamlet?

Horatio, unlike Hamlet, is calm, level-headed, reasonable, and cautious more often than not. He initially doubts the existence of the Ghost, but once he sees it, Horatio logically decides that Hamlet must be informed. When the Ghost beckons Hamlet to follow him, Horatio is fearful that the Ghost may be an evil spirit, and he warns Hamlet, who recklessly ignores any potential danger.

Hamlet "puts an antic disposition" on, thereby pretending to be someone he is not, but Horatio is "not passion's slave." He is a stalwart friend, a reliable source of information for both Hamlet and at the end of the play for Fortinbras when Horatio explains the carnage. Although Horatio loves his friend and at first wants to follow him in death, Hamlet convinces him that Horatio must live to tell Hamlet's story to the world. Horatio is the only person whom Hamlet trusted enough to share his plans with before he eventually revealed to his mother that he wasn't actually mad.

As in many of his plays, Shakespeare has included a young man about the same age as the protagonist who shares some traits with him but contrasts in others. Horatio is a student as Hamlet is. His role is to serve as a confidant to Hamlet and to offer advice. As a foil, he calls out attention to Hamlet's main character traits as well.