You can unlock costumes for the characters, and those two are some of them. Most of the costumes come from levelling up a shop to max in Norende, but the Vestal Garb and Bravo Bikini are obtained through different methods. To say any more on the Vestal Garb would be a massive spoiler, and all I will say about the Bikini is that it's in Florem in Chapter 5, again because of spoilers.