the person in question; the individual concerned; the person directly involved in an issue or case.
事件の当人に直接連絡を取った。
I contacted the person involved in the incident directly.
その問題の当人は今出張中だ。
The person concerned with that issue is currently on a business trip.
当人の意見を聞いてから決めよう。
Let's decide after hearing the person's opinion.