Thieves are great, easily in my 10 favorite FE classes. Great dodge tanks, stealing without keys and their promotion to assassins are good too. Altough in FE6 I only ever use Astol, Matthew and Legault tend to fall behind in combat once Jaffar joins and they have pretty shit defenses plus their promotion only grants 1 strength. But Thieves have great characters in them like Matthew, Legault, Colm and my favorite non-promoted thief Gaius (if we are talking about Assassins too then Jaffar is my personal favorite Thief/Assassin in the series)