The Final Fantasy 25th Ultimania art book has a good refresher, showing off some Tetsuya Nomura sketches of Final Fantasy XIII characters. The man is an artist and has talent to spare—and then some. For more sketches, have a look at FinalFantasyFXN.net (link below). Great stuff.
FF XIII Character Sketches from FF 25th Ultamania [FinalFantasyFXN]