The Druid is an interesting and powerful class. It usually has high Magic, and can use Earth Magic. This is in addition to a choice of Arcana, Staves, or Dark Magic. Though the class itself is in other Fire Emblem games, the class is very different (Due to its focus on Earth Magic). Most Druids have the ability to use a unique Avatar animation.
Andrea can become a Druid once she gains enough experience. Muzi starts out as one in chapter 18, though the strongest playable druid is definitely Steve. However, he is extremely difficult to summon unless you know what to do. If you do get Steve, you will probably want him in every battle.