Heartless

Heartless is a special perk in the Fallout: New Vegas add-on Old World Blues.

Obtaining
In the initial dialogue with Dr. Klein as part of the quest Welcome to the Big Empty, the Courier finds out that their brain, heart and spine have been removed and replaced. This perk represents the replacement of their heart. While technically having these perks from the start of the add-on, they are unlocked in the course of the conversation.

Effects
Immunity to poisoning, adds a +25% bonus to healing items and -50% to Critical Chance for robots against the player character.

Behind the scenes
Heartless is an adjective associated with cruelty or a lack of remorse. Despite it's vernacular meaning it has no bearing on the perk's effects.