Lifegiver

Livegiver or Life Giver is a Fallout, Fallout 2, Fallout 3, Fallout: New Vegas, Fallout 4, Fallout Tactics, J.E. Sawyer's Fallout Role-Playing Game, and Lionheart perk.

Fallout, Fallout 2, Fallout Tactics, and J.E. Sawyer's Fallout Role-Playing Game
Every time your character gains a level, you gain an additional four Hit Points for each rank of this perk. With two ranks of Lifegiver, that's +8 HP each level, which is more than 10 points in Endurance would give you.

Although it's not clear from the description, Lifegiver grants +4 HP for every level gained, including the level when the perk was taken.

Fallout 3, Fallout: New Vegas
You gain an additional 30 Hit Points.

This perk also gives you unique dialogue choices when talking with some specific characters, like Tree Father Birch in Oasis.

Bugs
In Fallout: New Vegas, the health that Lifegiver grants is excluded from the 50% calculation for Ferocious Loyalty.

Fallout 4
In Fallout 4, this is best as an early-game perk. The math is such that once you reach Level 32, you would have been better off just putting 1 more point into Endurance than in getting a level in the Lifegiver perk. However, the benefits of +20 Health early game can be considerable.

The health regeneration provided at rank 3 does not function in combat (i.e. when there are enemies nearby).

Lionheart
With each rank of this perk, you gain an additional 6 hit points.

Życiodajna moc Фонтан жизни Lebensspender