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.
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. 
Cath remains on Arthur's head throughout the Great Fight Festival arc.
Cath is still with Arthur when this, Nanashi and other Camelot knights fight against a horde of Gray Demons. When hearing it speak, Arthur is surprised and Nanashi affirms that Arthur would not die while Cath is with him. When the group is forced to retreat before the arrival of a Blue Demons horde, Cath agrees with Arthur that he must become stronger.
Cath along with Arthur is hidden in an underground shelter with the other survivors of Camelot. When Arthur arrives with Orlondi, Cath launches on this and keeps quiet about showing off.
Cath accompanies Arthur when he decides to go on his own to the castle of Camelot and recover the holy sword Excalibur. Being caught in the Marchen Maze of Peronia, Cath takes advantage when Arthur is attacked by a Gray Demon to approach Peronia from behind and apparently devour her.
After entering the castle, Arthur and Cath are surprised by Zeldris and Cusack. Cath decides to give Arthur the possibility of running away by levitating several objects around and attacking the demons with them. However, Cath is defeated and seriously injured.
When Arthur is able to wield Excalibur, he gives several cuts to Cusack to retrieve Cath and take him out of the castle. Arthur apologizes to him for making him suffer, to which Cath tells him he is hungry and Arthur says they will eat when they are finished.
After Arthur failed to defeat the demons, Cath points out that his magical power is still not awake and that he will not win if it does not wake up. When Arthur suffers the effects of brandishing Excalibur, Cath sends a telepathic message to Merlin warning her that Arthur will die.
When Merlin manages to remove Cusack with her Icicle Castle, Cath is teleported with Arthur to the Boar Hat in Liones. Hawk is alarmed to see him hurt but Cath insists that they must worry about Arthur. However, Arthur falls victim to Cusack's Resonant, piercing his own heart with Excalibur. Cath looks horrified to see such a fact.
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.
- Cath would beat Hawk in a fight.