誰か
だれか · dareka · JLPT N5 · noun
person
誰 (dare, who) + か (question particle) = some-who, i.e. someone
Adding か to a question word turns it indefinite: 誰 who → 誰か someone, 何 what → 何か something, どこ where → どこか somewhere. Learn the pattern once and you get a whole family of words.