The Visitor Event Guide/Discussion.

That be me. You take less damage the more defense your armor has, and the more to your main stat it adds the more damage you deal to aliens. However it coming into play during the boss is nothing more than a theory.

I think this has to do with number of upgrades, I've been adding only Connection Cells to my Page's armor and her damage to aliens has increased. Though they also add defense, so I don't know if that might also work for defense.