Mr. Purrface is a cat that aids Ramsey in the fight against the Rat King battle during the Animal Invasion event. Ramsey tends to go down to the rats in that fight, but Mr. Purrface's abnormally high defense allows her to dodge attacks and clean up the fight. Unfortunately, her low HP of only 30 means you may not want to take too long killing the remaining Rats, should they take out Ramsey.
In the 2.1 Patch, Mr. Purrface is used during a chain of quests to fight off rats that are infesting the outpost. Later in the chain, several characters attempt to groom Mr. Purrface, where she responds by attacking.
Mr. Purrface is also Ramsey's beloved pet. As expressed in the first cutscene of the game, Ramsey has a deep love for cats, particularly Mr. Purrface. He is distraught when she appears to have been run over in the mission where you move the hovel.
The buffed version of Purrface has damage resistance 10% 10% 10% 10% 10%.
For the "god-mode" version of Purrface see: Test Unit