Cath is a cat-like creature that appears to be just a small cat head with a tail. Its fur is white with red markings.
Not much is known about Cath's personality, as it has remained mostly silent other than sneezing at times. It prefers to stay on Arthur's head (and later his shoulder), and has refused to leave Arthur's side. Cath has the ability to talk, and has stated it is happy that Arthur is growing stronger. Cath appears to enjoy chewing on Arthur's hair and his cape.
Not much is known about Cath's history or how Arthur managed to find it after training in Istar.
After Arthur and Gowther have completed their training and return to the group, Cath is on top of Arthur's head. Merlin asks Arthur what the strange object on top of his head is, and Arthur and Gowther merely reply that it is a mystery. 
After leaving Istar, Gowther tries to remove Cath from Arthur's head, but cannot do so and only ends up hurting Arthur. Gowther notes that Cath seems to be stuck to Arthur. 
Great Fight Festival arcEdit
Cath remains on Arthur's head throughout the Great Fight Festival arc.
Cath's abilities and purpose are currently unknown, but Nanashi has stated that Arthur will not die as long as Cath remains with him.
- Cath's name is a reference to Cath Palug, a monstrous cat of Arthurian legend.
- Arthur named it Cath after the sneeze it made.
- It refers to itself in the first person as "Poku" presumably an old pronunciation of the first person, "Boku".
- According to the databook(s):
- Special ability: Not letting go of Arthur no matter what
- Charmpoint: His roundness
- According to his power level, Cath has the highest amount of Spirit measured accurately thus far in the series.