One is the main antagonist of TPOT. She is the first real villain in BFDI. She has stalked the contestants, made deals with said contestants, and caused a storm, which either killed or kidnapped Leafy. She isn't as strong as Two, despite also being algebralien.
Anyways, I love them. They are a very silly character. I wonder if they want revenge over Two, want to steal Two's power, or both!
TPOT 12: A Vague Theory About One