You, me, and this other guy

17 year old, Kristina is irritated by her annoying neighbor Eric. Eric is the same age, attends the same school, and always does the same stupid things to annoy Kristina. Such as egging her car all the time.
Kris and Eric never got along. They say rude remarks to each other, and even hit each other.
One day, the two enemies end up trapped together in a small room with some other unknown guy.
The two are already fighting two seconds after finding the locked door.
This "other guy" comes to be a mental helper for the two enemies and unfolds a secret between them.
This unfolding leaves them thinking. Will they think even worse of each other? And hate each other more? Or will something hit them and bring them together?
What has this mystery guy done? Without even reveling himself?