어느새: before one knows 어느새 가을도 벌써 끝나간다; Even Autumn is almost over already. (surprised how fast the time goes by, upon realizing it all of a sudden) 길고양이에게 한동안 먹이를 주었더니 어느새 가족이 되어버렸다. I fed a stray cat for a while and he became a member of the family. (It somehow happened naturally) 어느샌가 is very similar to above, but emotionally stronger. And no, 누구 & 누군가 are vastly different as you must already know.