Answer: Family relationships, sibling rivalry and revenge.
The Ghost is an important character in Hamlet, as he greatly contributes to the development of subsequent events. When the Ghost first appears in Act I, he informs Hamlet of several situations. He discusses family relationships as he talks about Gertrude, King Hamlet and Claudius. He also talks about sibling rivalry when he discusses how Claudius took his life, his wife and his throne. Finally, the Ghost tells Hamlet to seek revenge.