Rat is a megalomaniacal rat, who is frequently critical of the strip's style and artwork, as well as the other characters in the strip and all living things. He believes himself to be more intelligent than he actually is, and thinks more or less anybody else is stupid. He tends to aggravate people, particularly his intelligent friend Goat, and is easily aggravated by his naïve, dim-witted housemate Pig.
Rat is also a con artist, as he is often taking up professions he knows almost nothing about and charging people a lot of money, often insulting or confusing his customers in the process. At times, other companies make the mistake of hiring him, in which case Rat is usually rude to his customers and employers.
In the comic's history, Rat has hosted several jobs for varying lengths of time. Some of them include a Marriage Counselor, a Book Store Information Guide, a Feng Shui Expert, and an Oil Company CEO, among others.
Rat's current employment is a server at Joe's Roastery, a cafe that allows Rat the opportunity to be constantly rude to the "coffee crowd." The management discussed firing Rat, but it was decided that due to his "minority status" that it was too dangerous to do so - Rat promptly started to exploit this decision.
He has also made various forays into the world of literature, and Sunday strips often depict him typing novels. These novels usually are about a character called Angry Bob, who, in every book, finds true happiness, followed by an untimely death. Rat also has a comic strip, Dickie the Cockroach, about a cockroach who ties people up and slaps duct tape over the mouths of people who say stupid things (though originally he pulled people's heads off and put them in his closet). Recently he has begun writing children's books, starring the character Danny Donkey (Rat even made a doll of Danny who mysteriously came alive). He has made references to poetry many times too, although he has almost never written a poem himself.
While Rat is often coldhearted or mean to his housemate Pig, he has been shown to be kind at times, saying nice things to him (although quickly covering it up with his usual attitude). He sometimes lies to Pig to spare his feelings, although not very often. It has also been revealed that Rat has a soft spot for Pig's sister, Farina, a germaphobe who lives in an air-filtered bubble. She, however, had on-and-off feelings about Rat, often breaking Rat's heart and dating other characters.
Stephan Pastis, the strip's author and formerly a lawyer, started drawing Rat in law school, often killing him off for the sake of humor. In later attempts to get syndicated, Pastis teamed Rat up with Poe, a being that didn't resemble anything. The strip, titled Rat, was too depressing for the syndicates, and was turned down.
After some rejection, Pastis eventually wrote Pearls Before Swine, which was an attempt to revive Rat with a less depressing tone. Poe was replaced by Pig, and was syndicated by United Media on December 31, 2001.