Antonyms for the phrase "are one's side" include "are against you," "are in disagreement," and "are opposing." When someone is not on your side, they may be working against you or holding different beliefs or opinions. It is important to try and understand the other person's perspective and address any issues or conflicts in a constructive manner. This can involve active listening, respectful communication, and finding common ground. When working towards a common goal, it is crucial to consider all perspectives and find ways to work together to achieve success.