If you mean 'this' as in someone being in the same place as you, or in a photo you are pointing at, for instance, then you'd use the same sentence to ask a third person about him/her. :)
But in case you are thinking about ''Who's this?'' in the sense of 'Who is this/it (that I am talking to)'' while you are addressing someone directly, then you'd have to use the second person (singular or plural, depends on the level of formality)
Hope this helped?