A person who plays a crucial or decisive role in an organization, project, or situation; someone whose actions strongly influence the outcome.
このプロジェクトのキーパーソンは彼女だ。
She is the key person for this project.
警察は事件の背後にいるキーパーソンを追っている。
The police are tracking down the key person behind the case.
取引先の紹介で、キーパーソンに直接会う機会を得た。
Through a client introduction, I had the chance to meet the key person directly.