The Rat Princess is a character from Tommy and PJ who is further classified as one of the Santas of the Intense Universe. She is best described as a tan-colored rat which is personified only to the extent that she can talk and make human-like facial and bodily expressions. Her dress is a small cape and a coronet for her head.
The Rat Princess was created as part of the lore of the Intense Stories version of Venus by Oren Otter and her story is told in the original Tommy & PJ Christmas story arc, which was included in Wired for Christmas. Her story is also summarized in the Santas article.
Exactly how rats came to live on Venus is not known, although the popular legend suggests that they may have stowed away with the earliest settlers from Earth in the time of the patriarch-king Mier, before the cataclysmic world war which decimated space-faring technology on Earth until the 20th century. This suggests that in the intervening years, rats born on Earth became less intelligent, or at least unable to communicate with anyone but themselves.
As for the Rat Princess, it is also not known how it was that she came to be immortal, since she was born long before the present time in the context of most Intense Stories story lines.
Her only known family is her father the Rat King, though she doubtless had many siblings. PJ is an admirer, for obvious reasons, though her work takes the place of any formal relationship.