Phil is the chosen on in a prophecy,
with powers to save the worlds
of monsters and men. Dan is just
the sidekick with a hate for violence.

Dan is a monster. Not one who hides under
beds (usually) or with claws and fur,
but one nonetheless. Classified as such because
he is the Invisible Man. Other monsters, 
real ones, exist - vampires and werewolves
and dragons, but few know that they
do. Their world is entirely separate
from the human one. 

At least, it was. A new prophecy emerges
warning of a war between humans and
monsters, and of the one who can stop
it. His name: Phil Lester. It becomes
Dan's job to protect him until he comes
of age, and to stay out of sight until
that time.

Bonus(?):

Phil has the ability to"detect" monsters, and to stop their
powers. Accidentally, he gets rid of Dan's completely.