Isa is the Penguin Princess. She is the only student from the school who does not live underwater; instead, she lives in the Penguin Kingdom in the South Pole (which is considered to be between Salacia and Dryland). Isa was afraid of water, but with the help of Santa Claus, she overcame her fear and today she fondly wears a small Santa hat on her crown.
The origin of Isa's Santa hat and initial fear of water is elaborated in the short story Isa's Gift. Because Isa was born out of the water, she was too afraid to get into it even though her penguin friends have been swimming their entire lives. One Christmas, Isa received a magical Santa hat and when she put it on her crown, she found that her fear of water was gone, but she could not take the hat off otherwise she would drown and die. Around a year later, just before Christmas, Isa finds herself unable to explain in class what she intends to do with her hat as it is a Christmas tradition in Salacia for families to give the presents they received the year before to others. Later, after she explains the origin of her hat to Polvina, Ester and Tubarina, Tubarina and Ester begin fighting over it and it gets ripped. Isa sees this and begins choking, but she recovers a few minutes later and discovers that she is able to breathe underwater without her hat, proving that all she needed was a little courage. The next day in class, Isa tells everyone that she intends to give her torn hat to a penguin named Mr Penpen and that she wants another Santa hat this Christmas so she can remind herself that she has the courage to swim and live in the water.
In The Golden Penguins, Isa's mother, the Penguin Queen, loses a pair of golden penguin-shaped earrings near the school when she met up with Isa. Isa spent the next couple of days trying to find them, but unknown to her or anyone else, Ester had found them and was planning to keep them until she decided to give them back to the Penguin Queen the day after.
In The Big Chill, Isa and her penguin friends are playing on a large iceberg that had floated to Salacia, not realising the freezing effect that the iceberg is having on the other fish. She shrugs off the girls' request for help, stating that the iceberg will melt "sooner or later", but when the penguins begin to get sick from the warmer weather, Isa and the penguins follow the girls to a cliff leading into the Abysmal Kingdom, allowing them to recover from the freezing currents. Returning to the iceberg, the girls notice it beginning to melt and break, so they and the penguins jump and skip on it to quicken the process, allowing the other fish to recover.
In Deep Freeze, Isa laments not being able to visit the penguin colony with her parents during hatching season. Later on, as they are playing with some dolphins, Isa and the girls go to the aid of a penguin, who is on his way to the colony after he hurt one of his flippers escaping from a Drylander net. After evading some Drylanders, Isa and the girls take the penguin back to the colony, where he swaps places with his partner to look after their egg. Just as they are about to leave, they see Isa’s parents on the colony as well, but they successfully evade them by blending in with the female penguins who are going back to the sea.