Horatio is Hamlet's closest friend. He comes to Elisnore from Wittenberg for the funeral of the King. Hamlet trusts Horatio, who tries to provide support and guidance to his friend during his time of grief and crisis. He fears for Hamlet's sanity and health and worries that all the stress of the ghost's appearance and Ophelia's death will lead Hamlet to kill himself.
"Hamlet professes his faith in Horatio and praises his qualities of judiciousness, patience, and equanimity."
"Horatio enjoys the absolute trust of those who know him: it is Horatio whom the guards ask to witness the appearance of the ghost, it is Horatio with whom Hamlet trusts his suspicions regarding Claudius"