Bess is Dr Gilbertson's sister's cat from Postman Pat and the Identical Cats
Bess looks exactly the same as Jess, except for the lead an blue collar. When she escapes, everyone in Pencaster and Greendale think that she is Jess; so that when she pops Nikhil's balloon, Jess is blamed for it. However, unlike Jess, she is scared of dogs and likes catnip.
Julian initially prefers Bess to Jess stating "she's way more fun then Jess", however soon changes his mind after she escapes.
Bess and Jess Become friends.