"Who is the other person?"
Translation:다른 사람은 누구예요?
This is low formal speech. High formal speech ends in -ㅂ니까? but "regular" formal speech has no special ending, just 요.
Informal- -야? Formal- -예요? More formal- -ㅂ니까?
They mean the exact sane thing, they're all just in different formalities of speech. Korean is extremely dependentbon formality with all the nuances related to the language, but it'll all come with time :)
-니까 is the ending for interrogatory sentences of '-입니다' speech level. '-어요' speech level doesn't have different forms for imperative, declarative or interrogative sentences
Possible if the who is the subject: "누가 다른 사람입니까?"
Who as the subject in this case is quite weird and the corresponding Korean sentence would sound more like "Who is the different person?"