Piemur1 said: And why are there both types of Thieves?
While the changes are less drastic, there are actually at least 3 types of female warriors in the picture. Disgaea 1 (one chest belt, turned-down boots); Disgaea 2-4 (double chest belt, metal greaves); and Phantom Kingdom (sleeveless white shirt). There's also a Valkyrie from Phantom Brave (just down-right of the Archer in the middle-left).
And speaking of Archers, that middle-left Archer is from Disgaea 2-4, while the one in the top-left is from Phantom Kingdom.
There's also both Disgaea 1 and Disgaea 2-4 versions of Magic Knights and Mages here.