Ann is the other keeper of the Doom tree (along with Alan). She seems to be the weaker of the two because she runs out of energy more often than Alan. She poses as Alan's sister in Serena's school, while they are in fact lovers. Alan always lets her pick the "Cardians" to collect the energy from the humans for the doom tree.
When Alan met Serena he immediately began to like her and Ann was not happy about this to say the least. She was very angry about this until she first saw Darien, and immediately began to pursue him. She is always competing with Serena for Darien's attention.
This is Alan (blue hair) and Ann (pink hair) in their true form: