Armor

Armor reduces the an amount of the incoming damage dealt on you. The more armor you have, the more the damage is reduced, using the following formula.

Damage reduction = Armor/(Armor+100)

This works for repulsion armour, returning the correct 83.33% damage red.

This also means that you are never invulnerable, however, holding 1422 Tough times will cause a damage reduction high enough to round to 100%

Confirm: Unlike many games, all types of damage are reduced by Armor in Risk of Rain. This makes Shattering Justice more effective than such a debuff would be in a game where damage mitigation was split between multiple resistance types, such as Physical and Magical. The exact effectiveness of max -25 armor on enemies, however, is unknown because enemy armor values are unknown.