Christelle appeared in "Sally's Pen Pal", "Poster Boy", and "Serial Thief". Christelle is part of the "Good Manners Club".
In "Serial Thief", she was the criminal because she is a total neat freak and is worried about people messing up the school. So, she thought this was an opportunity to steal stuff from people that were not clean and make them go away.
In "Sally's Pen Pal", she and Sally showed a real relationship of enemies. Sally thought that Christelle was a big pain and gets on her nerves since she kept wiping out evidence, which upset Doowee as well. But at the end worked together to find the real criminal.