i don't entirely know, but dream world is the one with the which and weird door, and L is the witch? (i don't think that's spoiling the game to myself right)
The Dream World is where Celica's neighbor lives, in a slightly unusual appearance, as neighbors go. And L. isn't exactly a legitimate merchant, so she keeps her name hidden. Knowing the key characters in this game is crucial, so try to remember them. But not all of them. :-D
In any case, all sets of clothes and underwear can be obtained if you delve into the plot and game mechanics of the game.