What if “Nimble” was not only benefiting from light armor but also from low HP? By analogy of armor’s Maximum Fatigue threshold there might be Maximum HP threshold (for example 50) granting a higher bonus to damage reduction. Thus such baackgrounds as Ratcatchers, Swordmasters, Flagellants wouldn’t need to invest many points in health to survive or spending a perk on “Colossus”. Being lean and light with 1H weapons they are faster and harder to hit directly.
For all that the hardest task is surviving until “Nimble”.