Ullr is a Night Transformer. He takes the shape of a scrawny orange tabby cat with a black-tipped tail during the day and, come night, the black tail tip spreads out to cover him, turning him into a black panther with an orange-tipped tail.
Ullr belongs to Snorri Snorrelson, although during part of some adventures, he stayed with Jenna Heap. He is a Spirit-Seer like Snorri. Snorri looked through his eyes and helped save Jenna from Queen Etheldredda. In PathFinder, Snorri offers Ullr to Alice TodHunter Moon for protection when she, Oskar and Ferdie Sarn, Princess Driffa, and Lucy Heap, go to save the PathFinders and the people taken from the Castle.