A person who changes their personality depending on who they are with phrase
Answers
I'm not sure if our personalities change, but certainly our behavior does. The way the question is worded is philosophical, and therefore hard to answer. If you focus on how your behavior is different around certain people, then you might get an answer to what you're looking for.
Undoubtedly, we to act differently around different people and in different situations. For example, you may have a silly, playful side of your personality that comes out around family and friends. You also may be the CEO of a Fortune 500 company. Probably not a good idea for that practical joker to emerge in a board meeting! Get around old friends from childhood, and notice how quickly all of you fall into old patterns of communication and behavior. In a group of adolescents the "good kid" sometimes engages in negative. behavior due to peer pressure. The reality is we are all influenced by who we are around, where we are, and what we are doing.
The question can be an exercise in futility. It might make more sense to examine the various roles that he fall into based on who we are around in what we are doing. If you're comfortable with your behavior in these situations, then no problem. If you're not, take a look at why you acted in ways inconsistent with your values. If certain people influence you to act in ways inconsistent with your values, and take responsibility for your own actions. Ultimately, you have the choice and power to act in all situations you find yourself in.