If your character can mimic another character, you need to consider a number of factors.

What can your character mimic?

Can your character mimic powers? If so, what sorts of powers can your character copy?

To what extent can your character mimic things?

How does your character mimic another? What is the mechanism for mimicry?

Can physical powers be copied? (e.g. Nightcrawler's tail.)

How long can your character mimic another?

How fast can the mimicry occur?

How does the mimicry affect your character?

What are the limitations of your character's mimicry? What would be too much or too little for your character to mimic?

Are powers taken away from the target or just copied?

Are the powers full strength, greater strength, etc., than the target?

Does your character mimic the skills and psyche of the other person?

Does your character absorb the things from the target, leaving the target without them?

Note that this is a subset of shapeshifting.

Example Characters: Gideon, Mimic