Veiled Behind a Pretty Face is Mien's Daydream. It allows her to manipulate the light and sound of an environment to create the illusion of an impressive situation. Essentially, Mien is capable of making certain things that she does seem more impressive than how they actually are in reality.
While the activation of this Daydream is a conscious choice, its effects aren't, at least not entirely. Once Mien activates it, the Daydream veils the situation into one that is in accordance to what Mien would consider more impressive.