Heroes generally have two armor values: a front armor value and a back armor value. The back armor value is always lower, rewarding flankers and allowing for attacks from behind to deal increased damage.
Testing has shown that increasing armor past 80 provides no additional benefit; it is not known if armor scales off before this.
Degens such as Poison or Fire always use the highest (front) armor value, and do not benefit from attacking from behind.
Nearly all characters have the same armor profile: 15 front and 10 back. Exceptions to this are The Margrave (30/20), HK-206 (30/20) and Aisling (who has six armor values; one for each side and two more for front and back when Cador is in her blade. 25 front, 15 back, 15 left side, 25 right side, 35 front w/Cador, and 25 back w/Cador.)
In addition to many heroes sharing a 15/10 armor profile, many summons seem to have the same. Extensive testing has not yet been performed, but preliminary results suggest a 15/10 armor profile for many summoned creatures.
Currently, the armor values for Ramsay and Zandora have not been published, but it is likely that Ramsay's is 15/10 and Zandora's is 30/20.
Testing has shown that armor can go below zero and that negative armor values increase, rather than decrease, the damage taken.