Who had a boon that he gained half of the strength of any opponent he fought?

Vali received a boon from Brahma that he would gain half the strength of any opponent he fought. This made him nearly invincible in combat, as each opponent would effectively be fighting at only half their normal strength while Vali remained at full power. This boon was ultimately the reason why Rama had to kill Vali from hiding - a direct confrontation would have been disadvantageous.