The world is a confusing and uncertain place that's easier to navigate when things make sense. This is one reason it's natural for human beings to label the people around them. But no one wants to be labeled—told that they are a certain way or have to fit into a mold. This can be especially painful when the character doesn't believe the label fits (whether it does or not). When this happens enough, it can create frustration, insecurity, and a fear that could go a number of directions: the character may hide the aspects of their identity that fit the label, act out against the accusation, or surround themselves with people like themselves to avoid anyone who might put them in a certain box.

The character mimicking their peers so they won't appear to be different
Being extremely private
Giving vague answers when asked about themselves
Lying about personal aspects that would confirm a label
Embracing hobbies or activities that don't fit the stereotype
Hiding interests, preferences, and beliefs to avoid being labelled
Outwardly changing personality traits or values that would put the character in that box
The character becoming what they're accused of being (self-fulfilling prophecy)
Advocating for inclusion and against typecasting
Working very hard to disprove the stereotype
The character surrounding themselves with others who are just like them 
Increased self-monitoring to say and do things that will avoid labels
Avoiding people who have attempted to label the character in the pastassumptions 
Avoiding situations where the label would be obvious—e.g.