Ferocious Loyalty/Fallout: New Vegas

Ferocious Loyalty is a perk in Fallout: New Vegas.

Effects
Whenever your health drops below half, all companions in your party gain 50 damage resistance (not damage threshold). This means companions will ignore half of the base damage of any attack they receive. This combines well with high nerve to increase their survivability in hardcore mode.

Ferocious Loyalty will remain active as long as your health is below 50%, so the effect can be thought of as sustainable.