Clover is the first unicorn hero made available to us, and is introduced to us via the Clover the Rampant Forest Unicorn quest. Once we complete that quest, we will have Clover as a hero, and can explore her (his? I truly don't remember a gender being assigned to this character) magical abilities in all the various Places of Legend.
One of Clover's specialties, as an explorer, is finding Emeralds. Once you have Clover in your stable of heroes, you should never want for emeralds again. If ever you do, then you've not been sending Clover to the Fairy Falls often enough!
Speaking of stables, as you might expect of a unicorn, that's the kind of place where Clover lives. Once you place Clover's Stable, it will create XP for you, as long as you keep it supplied with Oats.