Yes, lone models are also "units". According to the rules (
pg 1):
Da Rules wrote:
"Models fight in units. A unit can have one or more models..."
So a hero is basically a one man unit, and so is a monster.
Cheers!
EDIT: This question is more important than it first may seem, as some scenarios award victory for units escaping or holding stuff, which means you can do these things with heroes and monsters too.