Die Hard

Die Hard is a perk in Fallout Tactics, Van Buren and Lionheart a skill in Fallout: Brotherhood of Steel.

Fallout Tactics
You don't give up easily; when your hit points get below 20%, you gain an additional 10% to damage resistance.

Van Buren
Davidson in Van Buren had this perk. Combined with his armor and his other perks, like Toughness, Davidson became quite powerful.

Fallout: Brotherhood of Steel
Each level of this skill increases your health for +20 (up to a maximum +200) hit points.

Points requirements

 * 1 Rank: 4 points (+20 Hit points)
 * 2 Rank: 8 points (+50 Hit points)
 * 3 Rank: 16 points (+90 Hit points)
 * 4 Rank: 32 points (+140 Hit points)
 * 5 Rank: 64 points (+200 Hit points)

Lionheart
When your hit points get below 20%, you gain +20% to all resistances and your armor class goes up by 5.