This article contains plot details from the latest book.
Princess Purryfins is Linh's pet murcat, purchased in Book 6: Nightfall from the shop Claws, Wings, Horns, and Things In Nightfall. In Book 5: Lodestar, Grizel had proposed a game of Girls vs. Boys in finding Keefe’s items of importance. As the girls had ended up winning, each of them was able to demand a favor from a boy. Tam, by default, knew his sister was going to ask for a pet. She later used this favor and chose her murcat which was noted could be very sweet, with Tam negatively remarking that their bites can make people lose fingers. Linh's murcat is part of an aquatic species, and has scales covering its body.
In Book 8: Legacy, Marella tells Sophie that Linh named the murcat Princess Purryfins. Marella states that she's "'Pretty sure Linh did it so Tam would have to say the words ‘Princess Purryfins’ on a regular basis.’”