Both boys go to Bayport High School, where they are in the same grade (Frank lost a year due to illness, although a specific illness is never stated).  They are both athletic; Frank is pitcher and quarterback; Joe shortstop and left-halfback (these positions are subject to change as the series progresses).

Both boys are mechanically advanced, piloting motorboats, airplanes, cars, motorcycles, and bicycles expertly. They own a boat themselves, The Sleuth, bought with rewards from their first cases.

Both Frank and Joe know sign language and Morse code. Their resumes would be littered with experience; both boys have dabbled in ice fishing, fly tying, deep-sea diving, fire-fighting, and fencing, among other things.

Frank and Joe both are evidently quite popular at school, have many friends, and each has a girlfriend. (Callie for Frank, Iola for Joe. After Iola was killed in the Casefiles, Joe adopted a new girlfriend, Vanessa.)

Currently, Frank and Joe both work for ATAC, a secret society founded by their father.
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Fenton Hardy
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Fenton, a NYPD cop turned private detective, is tall, dark, and keen-minded like Frank. He is a master of disguises (The Phantom Freighter, page 165) and codes. His age is uncertain, but definitely early forties. Fenton is described as "a handsome, strapping man." He is away most of the time on a case, yet is still able to have a secure family. Since many of the boys' cases sprout from Fenton's (at least in early stories), all three usually end up capturing a gang at the end of the book. The great detective has many a friend the world over. He employs several operatives, the closest being Sam Radley. Fenton founded ATAC, a society in which teenagers bring justice to the world.
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Laura Hardy
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The Boys' mother, Laura, is small and slim with blonde hair and blue eyes. She frequently takes a back seat to events due to Gertrude's loud and boisterous rantings. Laura is not very important at all, although she does help out in some books (The Secret Panel, among others). Laura is frequently described as "being away" or "visiting friends." She gave her good looks to Joe.
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Gertrude "Trudy" Hardy
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Raving, ranting, and sister to Fenton. The boys' aunt is tall, angular, sometimes portly, and pepper-tongued. A real spitfire! She was once popular at school with many admirers (The Phantom Freighter, #26, pages 158-59). Gertrude was also once engaged, but the groom-to-be was killed in a plane crash a few months before the wedding (Past and Present Danger, #168). At least once, Gertrude was said to be the boys' great aunt.
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Gertrude initially dropped in unexpectedly for long visits at a time, but eventually moved in with the Hardys (The Phantom Freighter). She had a cat named Lavinia, but it died. Gertrude sometimes "solves the case her way," unearthing clues herself and meeting informants, and, occasionally, criminals (The Phantom Freighter). She is an excellent cook, providing a constant stream of food for her busy family.

Gertrude is known as Trudy in the UB series, and has formed a love-hate relationship with Playback, the Boys' parrot.
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Chet Morton
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Chet is the Brothers' best friend, and Iola's brother. He is plump, freckle-faced, pug nosed, and jovial. Chet frequently shows up at the Hardy home to take advantage of Gertrude's latest cooking creation (and he himself is a master of the culinary art). He has many hobbies which coincidentally tie in to the current case of the boys. Chet also is on the Bayport High football team.
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Iola Morton
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Iola is Chet's sister and admired by Joe. Initially plump, she turns into a slender beauty, described as a "pixie."  Iola always thought Joe was the best guy in Bayport, but she was blown up in the Casefiles by a car-bomb. Happily, she still lives on in the Digests. Iola got a bit of time on the screen when she won an acting part in the TV show Warp Space (Trouble in Warp Space, #172). Iola has not yet figured into the UB series.
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Callie Shaw
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Callie is Frank's girl of interest, described as "vivacious." She is initially dark haired, but evolves into a blonde later on. In the early hardback books, Callie is said to live with her cousin Pollie Shaw in the city, although her parents live way out in the country. In the first Casefiles books, Joe and Callie seem to be at odds, but thankfully this was resolved--or perhaps dissolved--as the series continued. Like Iola, Callie hasn't shown up in the UB series.
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Vanessa Bender
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Vanessa came along quite some time after Iola's death in the Casefiles. She has blond hair and is very tall. Andrea Bender, her widowed mother, is an artist. Vanessa was created by the Ghostwriter Rick Oliver and is Casefiles-specific.
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Allen "Biff" Hooper
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Biff is tall and a boxer, with "muscles of steel." He was nicknamed after a favorite relative who was called Biff. He owns the boat The Envoy.
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Tony Prito
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At first, Tony is an Italian immigrant with English troubles. After a bit of time, that vanishes and he is described simply as "dark-haired, with olive skin." He owns a boat called The Napoli.
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Phil Cohen
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A small, dark-haired "Jewish boy," although the Jewish part fades with time. In the Casefile/Digest era, and in fandoms, he is known as an electronics whiz.
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Chief of Police Ezra Collig
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The Chief of the Bayport Police Force. Initially bumbling, plump, and red-faced, Collig later emerges as a canny, slender man.
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Detective Oscar Smuff
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Oscar is a detective on the Bayport Police Force and Collig's thick-headed stooge. Oscar eventually disappeared from the series entirely.
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Officer Con Riley
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Con also changes as the series goes by. He is slow and dumb, but eventually evolves into a smart, helpful officer. After this transformation, Con is a big help to the Boys. This is also the case in the Casefiles.