Knight Knight is a strong card to deploy near the end of a game. Though it has a rather hefty price tag with a lower attack, this card can restore HP to itself equal to the amount of damage it deals to enemies, including the enemy player. In other words, if nothing has Armor, it restores its ATK as HP.
There are 2 major things to keep in mind. First, this effect doesn't work while defending, so it's best to combine this card with Taunts so it can't be targeted by enemy monsters while it's vulnerable. Second, if its HP goes <= 0 before it heals up, it dies anyway.
Typically Knight Knight can kill at least three other cards before dying due to this ability.