1.
You use ~ 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 ~’s responsible to come forward.
CONJ
2.
You use ~ to indicate that the actual identity of the person who does something will not affect a situation.
You can have ~ you like to visit you...
Everybody who goes into this region, ~ they are, is at risk of being taken hostage.
CONJ
3.
You use ~ in questions as an emphatic way of saying ‘who’, usually when you are surprised about something.
Ridiculous! Whoever suggested such a thing?
= who
QUEST emphasis