Rat-in-a-Hat is the owner of Rat's shop and a friend to B1 and B2. He is busy, clever, and sometimes bossy. He says his catchphrase "Cheese and whiskers" when he notices that there is trouble around which makes him upset, and usually likes to trick the bananas and the teddies. But also, the bananas try to help him carefully until they make the things right.
The original series
Rat originally appeared as a purple rat with a pink nose, pink ears, and a pink tail, and wears an orange and yellow cap, an orange vest with yellow teardrops, a yellow bow, big green pants, a green watch, and yellow and white shoes.
The animated series
In the animated series, his appearance is arguably changed the most out of all the cast who appeared in the original series. He is now grey instead of purple, has a black nose and slightly shorter whiskers, and now wears new clothes such as a red cap, a cyan shirt with a collar, and blue pants with a belt. His size has also been altered to the size of a usual rat.
Rat sings his own song in the shop when he prepares to run it.
I'm a rat, I'm a rat, I'm a busy, busy rat.
I'm a rat, I'm a rat, I'm a clever, clever rat.
Strangely, while Rat at least had an antagonist side in most of the Bananas in Pyjamas series, in the 2014 folk tale Banana Splits, Rat is clearly portrayed as a villain who shows no interest in taking good care of his daughter.