What determines defense increase in heroes?

When a hero gains a level, what is it that determines how much it's defense is increased by? I think it's agility, but I'm unsure.

It is agility, you can edit how much agility you need per defense in gameplay constants. (In addition to the base defense)

ok : ) cuz i had a lv 18 hero with lv 6 spiked carapace and he had 19+13 defense! >< talk about uber! Thanks, I looked in gameplay constants but wasn't sure if what I had found was what I was looking for (it was)

Notice that every hero starts from a -2 defense if he has 0 agility.

EDIT: yes, thanks Griffen, it's -2

-2, +0.3 armour per agility point. May be changed under Hero attributes under gameplay constants.