"Look who's here!" => (어머) 이게 누구야! (female) / (어/야) 이게 누구야! (male) - in casual speech. => (어머) 이게 누구세요! / (어) 이게 누구세요! - in polite speech. 이게 누구야 is literally "Who's this!" but conveys the sense of "Who's this! (What a surprise!)" or "Look who's here". It's usually said with a distinctive intonation expressing a surprise.