Used with a negative verb to emphasize that not a single person does or is something. Stronger than 誰も〜ない.
Not a single person could answer the question.
No one agreed with his opinion.
Standard kanji-kana mixed spelling.
Common variant with ひとり in hiragana.
Variant with だれ in hiragana.
Combination of 誰 (who) and 一人 (one person), used with a negative to mean 'not even one person'.