The Rat

There's a new syndicate in town, planning a big score. The heroes have been given twenty-four hours by Da Chief to bust them - but they don't have any leads.

Better go see The Rat.

The Rat is an informant who always has the information, and he's willing to give it to the heroes, for a price - whether it's money, the expectation of future favours, or even just the heroes' blind eye to his shady dealings. In the criminal underbelly, he's usually a low-level bottom-feeder at best, and is often a bit of a slimy, whiny creep, but he has his finger on the pulse and every scrap of information about what's going down comes through him, and he saves it for exactly when it'll come in useful for him. He's often a pawnbroker (read: fence for stolen goods) or a bartender, or some other such profession which often features some kind of contact with criminals. In older works, he may be a Street Urchin of some kind. Usually lives in The City Narrows.

He usually likes to treat the heroes' search for information as a bit of a game; a request whether he knows what's going on will be met with a faux-innocent "I know lots of things," until someone ponies up the motivation to get him to talk. If the heroes are in the mood to play along, then this will be a suitably high amount of currency; if not, they may just decide to bounce him off the walls until he squeals.

As such, he can be a bit of a punching bag for the heroes, but beware - he can only be pushed so far. Whilst he's usually a coward who wouldn't dare try anything with the heroes when they're on top, his loyalties are slippery, and if the villain gives him enough motivation he'll just as easily switch allegiance and sell out the heroes, thus prompting a possible The Dog Bites Back moment if the heroes push him too far. However, if and when the heroes get themselves back on top of the situation, The Rat will find himself with a lot of explaining - not to mention desperate pleading - to do.

The chances of The Rat betraying the heroes if someone makes him a better offer is near-certain, however, and all but the most genre blind of heroes know this - nevertheless, they still persist in keeping him around. In-universe, this may be explained by the fact that he's just so useful as a source of information. Outside of the fiction, however, it's probably because he's a fan-favourite character.

A variation that sometimes appears (particularly in Cyberpunk style stories) is the 'information broker' variety - a character who deals in trafficking information, usually of a sensitive or not-entirely-legal in a computerised format. Unlike the standard crime-novel Rat, the Information Broker may not be a criminal and may have a respectable above-board persona requiring that they conceal their more illicit activities. This type of character frequently appears as a protagonist.

Compare with the Mysterious Informant (an anonymous Deep Throat-like source who appears to give the protagonists some cryptic, fragmentary information before disappearing) and the Knowledge Broker (a source with a network of further information sources supplying all the information needed). Not to be confused with an actual rat, though of course in anthropomorphic settings of all kinds it's entirely possible for the two to overlap.

Examples of The Rat include:

Comic Books

Sin City has Weevil, mine of information and perennial Butt Monkey.
In some variations of the Batman franchise, The Penguin has come to fill this role after going semi-legitimate with his owning a nightclub.
Nervous Rex fills this role in Sinister Dexter, right down to the massive coward bit. He is also, however, a good friend of the protagonists, to the extent of getting free (or heavily discounted) protection from them.

Film

Leo Getz in the Lethal Weapon series of movies is an unusually sympathetic version of this character; described by Riggs and Murtaugh as "an annoying asshole with a lot of connections," he's a bit irritating but essentially a decent guy who actually seems to look up to Riggs and Murtaugh a bit, however badly they treat him. In the later movies, the three become friends, something of a Power Trio, and Leo ends up acting as Murtaugh's estate agent at one point.
A rare Sci-Fi version in Men in Black I and II. Jack Jeebs is an alien who sells stolen fake rolexes and some powerful alien tech who J and K harass and threaten to get info and tech.
The Rat in The Core is actually called 'Rat' and is an "information wants to be free" type – which he uses as a Take That against the government in the final shots of the movie.
Mr. Universe from Serenity.
In Osmosis Jones, a Flu Shot virus serves this role.

Literature

The Harry Potter series of books has Mundungus Fletcher, a selfish thief and con artist who is sometimes antagonistic towards Harry and his friends. Despite being a total crook he remains loyal to Dumbledore and is thus useful to the protagonists due to his knowledge of the criminal world.
Nobby Nobbs in the Discworld novel Night Watch.
On the Cyberpunk / protagonist side of things, we have:
- Hiro Protagonist and Y. T. in Neal Stephenson's Snow Crash, who make their living trading "intel" (usually stolen computer code).
- One of Robert Asprin's novels (possibly Cold Cash Wars) featured a character like this.^[context?]
Nick the Nose, an informant in the Two Minute Mysteries stories starring Dr. Haledjian. In the stories, he always lies about knowing things in order to get a reward.
The Finn in William Gibson's Neuromancer is compared to a rat by the main characters, and more or less fits the job decription.
In Robert E. Howard's Conan the Barbarian story "Rogues in the House", the priest of Anu who betrayed Conan at the beginning of the story.

Live-Action TV

Parodied in Police Squad!—Johnny the shoeshine guy acts in this role to Frank Drebin. However, as soon as Drebin's gone, someone from another profession entirely - a surgeon, a priest, Dick Clark, Dr. Joyce Brothers - will show up, asking for information - and Johnny will go through the same act again, eventually giving them what they want as well.
- Similarly, in The Naked Gun, Johnny answers some questions after Frank Drebin greases his palm... then Johnny asks why the information is so important, and Drebin doesn't answer until Johnny pays him. Drebin ends up leaving with the information he needed and an extra twenty bucks.
Buffy the Vampire Slayer: Willy The Snitch.
- Angel also had a sewer-dwelling demon on whom Angel frequently used to pound in order to get information.
The dwarf from Special Unit Two.
Piggy from Power Rangers SPD.
The actor Dick Miller made a career of playing characters like this. (One example being Fosnight from the |The Flash TV series.)

Video Games

Hammer the Supplier from Xenogears is literally a rat and fits this trope to a T. Complete with a Face Heel Turn because he's tired of being cowardly and powerless and jealous of the main characters' strength.
Ugarte from Quest for Glory 2 and 5.

Western Animation

Rattletrap from Transformers Animated, despite being an Autobot, is a little shady (even willing to sell an ID pass to a Decepticon). He's based on Rattrap from Beast Wars, making him a literal rat as well. As Captain Fanzone put it: "There's one in every town."
Argit from the various Ben 10 series.