Kathleen is a girl that is one of Morgan's apprentices and a selkie. She is the youngest of twenty sisters who are very protective of her. She is good friends with Jack, Annie, and Teddy and assists the latter in creating magic spells for Jack and Annie.
Kathleen is very friendly and kind and makes friends very easily, quickly befriending Jack, Annie, Teddy, Morgan, and Merlin. She also appears to be quite free-spirited and playful, possibly due to the fact that she is a selkie, creatures that can morph into seals, and therefore acts as such. Despite this, she is very serious when she needs to be and is quite respectful of the boundaries set for her, seen when she hesitated to travel with her friends to the cave that held the Sword of Light, where selkies are generally forbidden from going to. Overall, Kathleen is a sweet, charming young lady who will do anything to help her friends. During Danger in the Darkest Hour, her experiences in World War II changed her so that she felt hopeless and weak as she was unable to rescue ten children from the Nazis and feared she'd be responsible for their deaths. This also sapped her magical abilities, but the appearance of Jack and Annie who help her through non-magical means restores her confidence and her abilities.
Kathleen has long raven black hair reaching down to her waist and sitting in ringlet curls. Her eyes are stunningly blue, crossing between the color of the sky and the ocean with a bright twinkle. She wears a green dress made from seaweed, although it isn't very well woven. She is quite pretty and was once described as "the most beautiful girl he Jack had ever seen". She is also very attractive and is found in Merlin`s rhyme. Teddy probably likes the fact that she is also lovely when in her seal form.
As a selkie, Kathleen's primary abilities are turning into a seal and and turning other people into seals When in her seal form, she can speak the animal's tougue and communicate with other seals as well. She is also very talented in magic spells, as she used one to turn Jack, Annie, and Teddy into seals and helps the latter in finishing his own spells. This leads to the two of them creating spells for Jack and Annie to use on their own. She also seems to be a master of disguise, using various disguises alongside Teddy to help Jack and Annie with some of their adventures. She is not, however, very good at weaving or sewing. During Danger in the Darkest Hour, he hopelessness drains her abilities and she is unable to use them to save herself or ten young children. With her abilities or the Wand of Dianthus, she believed that she could transform them all into birds to cross the English Channel, but couldn't do it since the situation she was in and the horrors of World War II sapped her hope and will. The appearance of Jack and Annie helped her regain her magical abilities when they helped her rescue the children, even without magic. She displayed her renewed abilities by copying flyers of hope for Annie thousands of time as they flew over Normandy. Earlier, she pretended that she could do an invisibility spell to protect them from the Nazis, but it was really to boost Jack's confidence and she couldn't actually perform the spell.
Kathleen is very good friends with Teddy. The two of them met at a beach where they played in the water together before seeing Jack and Annie asleep on the shores. Since Teddy often can't finish his spells, Kathleen will offer to finish them for him, which eventually leads to them creating spells for Jack and Annie to use throughout their journies. Whenever Jack and Annie need their assistance, it's never one or the other; it's always both of them at the same time. Both of them are Morgan and Merlin's apprentices. They have a very strong friendship and love each other like brother and sister.
When Kathleen met Jack and Annie, Annie, immediately took a liking to her and started hurtling questions at her, such as whether she had any siblings and whether she had made her dress. Kathleen often assists Annie and her brother Jack in their adventures and even helped Teddy make a magic book of spells for them. Kathleen and Annie are very good friends, and the latter seems to look up to Kathleen as a role model.
When Jack first met Kathleen, he was at a complete loss for words and was unable to speak. Only when she smiled kindly at him did he have enough confidence to talk to her without sounding stupid. Kathleen quickly befriended him as well as his sister Annie. Throughout the book Summer of the Sea Serpant Jack was totally charmed by Kathleen's beauty and kindness. From then on, they have become good friends, and she and Teddy have asisted him and Annie in several of their missions.
It has been implied that Jack may have feelings for Kathleen, though hers are unknown. It's quite possible that she just sees him as a little brother seeing that she's much older than him. However, she may have started to return his feelings as in Super Edition #1 she hugged him and called him her hero when he and Annie risked their lives to save her and ten children from behind enemy lines in World War II. In return, Jack was shown to want to be her hero and was willing to risk his life just to help her out of a terribly dangerous situation.