Sometimes, a detective or police officer, no matter how great, simply needs help from time to time. It's no big deal, really. The Baker Street Regular is that child that finds himself in the situations with the hero, especially the dangerous ones.
Your first run in with the Baker Street Regular is likely while he is searching through the hero's home or office or stealing something to eat because he cannot afford it. If he is not related to the hero in any way, expect him to have no family, except for maybe a sibling or two, but he is almost always the eldest, and almost always male. If he is an orphan, after scratching his back a couple times, expect him to be forever indebted to you and to get you out of a sticky situation in the nick of time, but don't expect him to just leave. You're stuck with him.
After you manage to win the respect of the Baker Street Regular, he will be an invaluable tool and assistant to your group. Life on the street is going to make him able to hear rumors that you won't be able to while you are out on your investigations. He is always good at hiding, especially in situations where the Big Bad is nearby.
The Baker Street Regular is a pun on the Trope Namer, the Baker Street Irregulars, a mob of orphaned street urchins often employed by Sherlock Holmes to gather rumors, spy, and do various odd jobs. In return for their discretion and loyalty he paid what was, to them, an outrageous sum of money.
"By heavens, Holmes," I said, half rising, "I believe that they are really after us."
"No, it's not quite so bad as that. It is the unofficial force — the Baker Street irregulars."
As he spoke, there came a swift pattering of naked feet upon the stairs, a clatter of high voices, and in rushed a dozen dirty and ragged little street Arabs. There was some show of discipline among them, despite their tumultuous entry, for they instantly drew up in line and stood facing us with expectant faces. One of their number, taller and older than the others, stood forward with an air of lounging superiority which was very funny in such a disreputable little scarecrow.
See also The Informant, who is usually somewhat older and portrayed rather less sympathetically.
- Subverted in Black Butler. Doll, aside from not being the eldest (though that is justified due to her elder "siblings" being Genre Blind), gives Ciel help, covers for him, and fits this trope to a T before Ciel goes into an episode and orders for her death.
- In Detective Conan, Ayumi, Genta, and Mitsuhiko, the Detective Boys. They're even referred to as Baker Street Irregulars in an arc. They literally play this when they entered the cyberspace Victorian England AU in the Non-Serial Movie Phantom of Baker Street.
- As the plot approaches the end of Slayers Next, one of these attempts to pickpocket the party, but eventually joins them and befriends Martina. Subverted, as he's the Big Bad.
- In the Post-Crisis retelling of his origin, this is how Jason Todd first met Batman. He was trying to steal the Batmobile's hubcaps at the time.
- The Newsboy Legion are nothing if not this trope. Primarily they are this for the Guardian, but they also work with Jimmy Olsen and Superman. Guardian took them in and became their legal guardian in his secret identity.
- Marvel Comics had its own Newsboy Legion equivalent in Teen Brigade, a team of ham radio buffs (led by Hulk's buddy Rick Jones) who helped superheroes network. They were actually partially responsible for the founding of The Avengers.
- Dick Tracy, Jr, from Dick Tracy, is one of the examples of a street urchin who gets adopted. Also appears in the film.
- Modesty Blaise: Samantha 'Sam' Brown is an East End kid who attends a martial arts class run by Willie. She and her gang of street urchins sometimes fill this role for Willie and Modesty.
- Charlie Chaplin in Shanghai Knights.
- Short Round in Indiana Jones and the Temple of Doom.
- The street urchin Toto in American Ninja 2, who never forgets to demand money for service rendered.
- The kid Manco pays to keep tabs on Mortimer in For a Few Dollars More fits the bill.
- Averted with the Blue Blaze Irregulars in The Adventures of Buckaroo Banzai Across the 8th Dimension. They're not all kids, they're not all urchins or orphans, and there is (according to the book, anyway) a training program, required progress in education, and other requirements.
- Subverted in The Arrival when the kid who hooks up with the protagonist turns out to be a spy sent by the aliens. In fact he's an alien himself.
- A Walk Among the Tombstones. T.J. provides both street-smarts and computer literacy to the private eye protagonist, who doesn't have any computer skills.
- The Baker Street Irregulars from Sherlock Holmes, who inspired the trope name.
- They're of great help even in the first case Watson sees, tracking down a singular cab in the entirety of London.
- Deconstructed in the officially approved pastiche The House of Silk, when one of them gets killed, brutally, because they took a mission.
- Akechi Kogoro had several as well, including Yoshio Kobayashi, who later became famous for something else entirely.
- Solar Pons has the Praed Street Irregulars.
- Nobby Nobbs is introduced in Night Watch as one of these. Vimes sees Nobby spying on him for a whole slew of people, and begins paying Nobby to, in turn, spy on them.
- Sometimes members of the Beggar's Guild, although largely adults and therefore on the very fringes of the trope, act as this for Lord Vetinari as one of many information-gathering channels (it's implied that someone who may in some small way be connected to him spreads rumors that he pays for information, so that they'll come volunteer it to him instead of him needing to send someone looking). It's implied Vimes occasionally gets information from them as well.
- Ostap Bender from Ilya Ilf and Eugene Petrov's The Twelve Chairs employed them to track one of the titular, possibly treasure-holding chairs.
- Ben Fischer had apparently started as this in his Backstory in Hell House, before his powers awakened.
- In The Eagle and the Nightingales, Nightingale hires a group of street children to be her eyes and ears.
- In Honor Harrington while on a secret mission to Mesa, Victor Cachat employees a number of street urchins to serve as a spies and couriers. The Mesan organized crime bosses also employ lots of these. And Victor himself served as one to Noveau Paris' criminal elements in his youth.
- Varys Kingfisher, the royal spymaster in A Song of Ice and Fire, is implied to employ mute (because he had their tongues cut out to prevent idle gossip) yet literate (so they can read secret documents without stealing them) children who wander the sewers and low places of King's Landing as his "little birds."
- Maya from Garrett, P.I. started out as this, hiring out members of the street gang she leads (the Sisters of Doom) to Garrett for information-gathering tasks. Subverted when she grows up enough to decide she'd rather be his Love Interest instead.
- In the Heralds of Valdemar series, Mags rescues gangs of street kids forced to work as thieves for various cruel men and recruits them to unofficially be informants for the Crown (and officially as runners and messenger boys.)
- Game of Thrones: Lord Varys has an Irregulars network he refers to as his "little birds." These in later days fall into the employment of Qyber, the necromancing dishonored Maester put on the Small Council by Cersei Lannister.
- The Pretender episode "Back from the Dead... Again" has Bruno who gets recruited by Jarod to help him for the episode.
- Fittingly for a reimagining of Sherlock Holmes himself, Sherlock has Bill Wiggins, a heroin addict who doubles as a chemistry genius. The pilot episode introduces Sherlock's "homeless network," who are an obvious stand-in for the Irregulars.
- Dirk Gently knows a Hollywood Hacker who happens to be twelve. And gets paid with cigarettes.
- Kamen Rider and Kamen Rider V3 had the Boys' Kamen Rider Squad, a fanclub consisting mostly of preteen boys who would contact the heroes via radio if they spotted suspicious activity.
- Kamen Rider Super-1 had the Junior Rider Team, but they were more an example of Tagalong Kids, trying to investigate things on their own, getting captured, and needing to be rescued by Super-1.
- Kamen Rider Double, who actually is a detective, has a network of informants he calls the Fuuto Irregulars; however, none of them really fit this trope. The closest it gets is with Queen and Elizabeth, a pair of high school students who were already informants for Shotaro before coming to him with a case partway through the series.
- Billy, the page boy to William Gillette's Sherlock Holmes, who was adapted from a similar character in the original Holmes novels.
- Lilly in Ceville.
- The Black Ravens Gang in Professor Layton and the Last Specter is composed entirely of these. In a minor sense, Luke and Flora may also qualify, although they're not street urchins, Flora doesn't help much with the investigations and Luke's parents are still alive and well.
- Brynn from Dreamfall: The Longest Journey acts this way towards April.
- In Mass Effect, Thane would sometimes employ street urchins to gather intelligence for him.
- Clara O'Dea and her urchins in Assassin's Creed: Syndicate. Considering the Victorian London setting, almost certainly a tip of the hat to the Trope Namer.
- Kit Cloudkicker in TaleSpin. Molly can also apply in most episodes she's in.
- Sherlock Holmes in the 22nd Century has the new Baker Street Irregulars: soccer player Wiggins, the Eliza Doolittleish Deidre, and the paraplegic Tennyson (who communicates through electronic beeps only Holmes seems to comprehend ironically).