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 its ATK whenever it attacks an enemy monster. It doesn't need to kill it to heal up.
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.