Horatio is Hamlet's best friend and the most reliable character in the play. Everyone who knows him trusts him. In terms of what a good friend should be, Horatio is the epitome of a good friend. He even wants to kill himself when Hamlet is killed, but Hamlet begs him not to.
Laertes is different from Horatio in some regards. He is protective of Ophelia, his sister, and swears vengeance for his father's death. He may be jealous of Hamlet and certainly does not trust his honor with Ophelia since he warns her against him. He is too quick to believe Claudius when questioning him. He betrays Hamlet and conspires with Claudius against Hamlet. He is a character who has weaknesses amidst strengths, just like Hamlet. Both he and Hamlet show trustworthy traits at the end.
Fortinbras is very much like Hamlet in terms of his life his situation. His own father has died, and he wants to avenge his father's death also. His father's brother, his own uncle, is ruling Norway just like Hamlet's uncle is ruling Denmark. He speaks well of Hamlet in the end and grants him a noble soldier's burial; Hamlet has named him as the rightful successor to Denmark's throne. His role is to provide a contrast between Hamlet's questioning of the tradition of revenge killing and ready acceptance of it. The ironic unanswered question posed by Fortinbras' triumph in the end is whether rejecting the revenge tradition or accepting it is the right and noble course. Hamlet's death doesn't provide an answer for him or for us.