How do I protect my team's culture after a new hire changes it?

One new person joined, and the dynamic shifted in ways you did not see coming. It has shown you something a little unsettling, that the culture you built was more fragile than you believed. That is a real thing to sit with, and it does not mean you did anything wrong. Living cultures are always more delicate than they look. The instinct now might be to clamp down, to enforce the old way and push the newcomer to conform. Be careful with that. A culture worth protecting is not a set of rules to defend, but a set of values people share. If you get rigid, you can crush the very thing you are trying to save. The goal is to hold the values while staying open to the person. So get clear, first for yourself, on what actually made the old dynamic work, the two or three things you will not compromise on. Then have a direct, warm conversation with the new hire about how the team operates and why. Most people adjust when they understand the why. You protect a culture by naming it and living it, not by fencing it in.