My Characters nickname is Pop and i don't know what their real name should be?
Names for Pop
Poppy?
Pops?
Something completely unrelatable to 'pop' maybe (how did they get this nickname btw?)
Could be because they love poppies (those flowers) or because they speak the P with a popping sound for example.
Maybe consider names Robin, Jackie, Savan, you can call them after the flower poppy if that fits their personality.