Look who's here!
Used to emphasize how wonderful it is that someone is present or has just arrived. Look who's here, everyone—Uncle Marshal! A: "Well, would you look who's here! I haven't seen you in years!" B: "Hi Grandpa!"
Look who's here!
an expression drawing attention to someone present or who just arrived at a place. Bill: Look who's here! My old friend Fred. How goes it, Fred? Fred: Hi, there, Bill! What's new? Bill: Nothing much. Bill: Look who's here! Mary: Yeah. Isn't that Fred Morgan?