Are you still here ? : You would probably ask someone else this question if they were already here. You try to see that they are not gone.
Are you already there? : You probably ask the other person if they’re not there, but you wish they were there soon.
Are you already here? : you would use it in some situations.
Someone comes to your house, or comes to where you are, but you can’t see them. So you check to see if he or she is here. ” Where are you ? I don’t know where you are. Are you already there?
Or someone came earlier than expected. “Oh, are you there already? I wasn’t expecting you for five minutes.