Gremlin

Now, this is a different meaning from those ugly fairytale creatures.

The gremlin is simply a title to a side of everybody's personality. This side is rarely exposed to other people, and when it is, it is exposed under very specific circumstances. A usual reaction to experiencing someone's inner gremlin is a mixture of shock, disgust and sometimes amusement.

Example :

Somebody who always seems genuinely friendly and cheery -And even when she is unhappy, she usually laughs it off a few minutes later. And then- one day you go to an awesome concert and in the mosh pit she is punching and shoving like mad. Her inner gremlin is exposed!