Used when the identity of a person is unknown or deliberately unspecified, often with a nuance of mystery or suspicion.
It seems someone broke in through the window.
He was being followed by someone.
Mixed kanji-kana spelling; less common but still used.
Compound of 何 (nani, 'what') + 者 (mono, 'person') + か (ka, question marker). Literally 'what person?', used to refer to an unspecified or unknown individual.