Invisibility is an incredible power, but also a corrupting one. It also can allow a person to see the secrets of some one else. The dark things they hide from the world which they only speak of when they're absolutely alone... Christelle can ,however, make it so they are not truly alone. She has the ability to make herself invisible in a matter of speaking.
Basically what she does is phase out of our dimension just a little bit. This allows her to not only appear invisible, but also makes it difficult to detect her using the other four senses available to people. While in this 'invisible' state her voice is no where near as loud as a person's voice. However if she whispers something in your ear you will definitely hear it.
Her kind are believed to be the origin of the common ghost. Only seen on heard under the right conditions. They're also not much as pranksters which is good for the rest of us. Being practically unable to be heard or seen would make pranking really easy to pull off.