-지 않아도
This pattern means "even if you do not..." or "without needing to...". It is formed by adding -지 않아도 to a verb stem. In the story, "꼭 누군가와 함께 하지 않아도" means that being with someone is not necessary. You can reuse it to say that an action is unnecessary or that something is still true without that action. Example: "꼭 비싼 여행을 하지 않아도 충분히 즐거울 수 있다." = "You can still have plenty of fun even if you don't take an expensive trip.".
꼭 비싼 여행을 하지 않아도 충분히 즐거울 수 있다.
You can still have plenty of fun even if you don't take an expensive trip.
