Catherine Called Birdy is about a 14 year old girl who is being forced to marry by her dim-witted father. She- quite understandably -believes she isn't ready, so she scares away all of her suitors...
Aelis is Catherines friend who falls in love with catherines uncle, George. Catherine puts a curse on them and Aelis marries a king who dies. Then Aelis marrys Catherines horrible brother Robert, and...
She's an average sized, pretty, young lady with brown hair and grey eyes (You can see her on the cover of the book. Or just google "Catherine Called Birdy" and look on images.)