Units with this keyword are stubborn fighters who are aggressive even when defending. After this unit is attacked it performs its regular counterattack (if available) and can then eliminate one of its own figures to make one additional counterattack (whether the unit had an original counterattack or not).
Once per attack, Scrapper Units may remove a figure in order to conduct a counterattack against an enemy attacking Unit (keeping in mind the rules for Reduced Strength Units, if using). This may be in addition to a counterattack granted by an Ability, like Stalwart, potentially allowing the Scrapper Unit to counterattack twice. It is not necessary for Scrapper Units to be Stalwart in order to utilize the Scrapper Ability.