Whenever Gree attacks and does no damage (after damage prevention), put 1 damage counter on Gree.
Whenever damage from Gree causes one of your opponent's units to be discarded, draw a card.
Keywords
Stealth
Stealth: An isolated, static ability that means, "This unit can't be attacked unless it's tapped or has damage counters on it." An untapped and undamaged unit with Stealth can still be affected by alternative types of damage. (Such as Overkill.) As long as the unit has any number of damage counters on it, it can't use Stealth.