You use whoever to indicate that the actual identity of the person who does something will not affect a situation. You can have whoever you like to visit you Everybody who goes into this region, whoever they are, is at risk of being taken hostage
Whatever person; any person who; be or she who; any one who; as, he shall be punished, whoever he may be
emphasis You use whoever in questions as an emphatic way of saying `who', usually when you are surprised about something. Ridiculous! Whoever suggested such a thing? = who
You use whoever to refer to someone when their identity is not yet known. Whoever wins the election is going to have a tough job getting the economy back on its feet Ben, I want whoever's responsible to come forward