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.
The person in question knew nothing about it.
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'.