What do you do when you realize one of your employees is a poor culture fit when you're a small startup and every employee feels so integral?
The firsts thing to try to do is reach out the employee and see if there are issues with communication or expectations that can be resolved to help the employee be a better fit. From there if the problem isn't solvable I would start to slowly isolate the employee and manage them out. If they do anything that impacts the integrity of the business or are preventing you from properly moving forward, terminate.