Who is marcellus in hamlet?

Marcellus is a character in Shakespeare's play "Hamlet." He is a guard at Elsinore Castle and is one of the first to witness the appearance of the ghost of King Hamlet. Alongside Horatio, Marcellus helps to confirm the ghost's authenticity and later joins Hamlet in his quest for revenge against King Claudius. His famous line, "Something is rotten in the state of Denmark," underscores the theme of corruption within the play.