Greenleaf is the first dragon hero you generally have access to in the game. I say generally because, it is possible that someone may come in during a seasonal quest, such as the 2014 Easter questline, that will give you a dragon hero before level 20. But barring any such special circumstances, Greenleaf becomes available to you at Level 20. Not only is Greenleaf a handy hero to have around, Greenleaf's Nest is also handy, as it will take in Cream, and produce XP for you on an ongoing basis.
To get Greenleaf as a hero, you must purchase him with Pearls earned from Liang the Dragon Merchant. For most players, Greenleaf will be the first hero they purchase with pearls, as his price is so low - a mere 5 white, 2 black and 1 gold pearls.
Quests given by Greenleaf:
Greenleaf's Big Adventure