Girl is extremely average with dumb problems and low self esteem, so the book appeals to the female teenage crowd.
Boy is preferably a supernatural monster.
However, Boy is still sexy despite being a freak of nature.
Boy goes through some deep internal conflict and ends up falling in love with Girl.
But before anything serious happens, another boy falls in love with Girl.
Boy goes crazy with jealousy.
Stuff happens, several people die, and revelations are made.
Boy and Girl chase after each other a lot.
The hearts of the teenage girls melt.
But don't worry!
Everything is okay.
For in the end, Boy and Girl fall in love and get married and live happily ever after.
They have lots of mutated monster-baby kids together.
