Student or Learner
Who is this girl?
The one with red skirt.
Sorry,I don't know her.
Any comments please?
'This' is appropriate if the girl with the red skirt is standing directly in front of you, or if you're pointing to the girl in a photo. 'This' implies immediately in front of you or something you can touch and show to other people. If the girl were standing across the room and you point at her asking who she is, "who is that girl?" sounds much more appropriate. Even if it's a photo of the girl, pointing to the photo you can also ask "who is that girl?" and it sounds fine.
Maybe this is just the way I use it, but to ask "what is this?" I would expect that the object was in my hands or at least right next to me so I can touch it, or in a photo or picture that I can point right at the object.
(not a teacher, just a language lover)