noun
the person in question; the very person
Emphasizes that the person being talked about is exactly the one relevant to the situation, often used when the person themselves is unaware or denies involvement.
当の本人は何も知らなかった。
The person in question knew nothing about it.
噂を聞いた当の本人が怒り出した。
The very person who heard the rumor got angry.
Compound of 当の (とうの, 'the very, the said') and 本人 (ほんにん, 'the person himself'). 当の is a fixed modifier meaning 'the one in question'.