Cossette is a petite girl with very pale blonde hair which is kept in a twintails. Her eyes are deep blue and no visible pupils, which means Anridia. She has a pale to fair skintone. She is wearing a sailor dress with double skirt and white stockings with simple khaki colored boots. She is also wearing a brown belt, filled with bandages and pills. Her arms are covered in bandages and wearing an eyepatch. She is carrying a large gun-blade, named Castle-Cloud.
According to Lisa, she is childishly sweet and hyper active. She is also smart and shy. She is also showned to be reckless. She is also obedient to other people. She have the habit of stretching and making ballet steps. She dislikes the people who gives agony to her sister, which are Ron and Ægir.