Since being published in 1927, the Hardy Boys have developed a full cast of characters in Bayport and around the world, some of who lasted the entire run of the series, and others who have since been phased out. Below are the most frequently seen of the recurring characters.

''Casefiles''-only characters are noted in their descriptions.

! The Hardy Family
!! Frank Hardy
'''First appearance''': #1 ''The Tower Treasure''

The older of the Hardy brothers, 18 as of 1974. Dark-haired and methodical, Frank is the brains of the two, preferring to use logic and thought in solving cases over more physical means. He's no pushover in a fight, though; he keeps his slim frame in good physical shape and holds a black belt in karate.

* BadassBookworm: Equally at home with research, computers, and getting out of dangerous situations with karate.
* BigBrotherInstinct: Averted; Frank does worry about Joe, but he knows his younger brother is capable of taking care of himself.
* BornDetective: Raised by former NYPD detective and now current PI father, Fenton.
* BrainsAndBrawn: The brains half.
* ChildhoodFriendRomance: Case 3: ''Cult of Crime'' centers on rescuing Holly Strand, [[RememberTheNewGuy supposedly]] a childhood friend of the Hardy Boys. Frank mentions that, before he met Callie, he thought he and Holly would have probably fallen in love as they grew up. Frank has moved on, and Holly gets PutOnABus at the end of the book, but her feelings for Frank is still strong.
* DistressedDude: Frequently kidnapped as part of the story, sometimes with Joe, sometimes solo.
* GuileHero: Even Joe can occasionally become a pawn when Frank plans to outsmart their enemies.
* NoodleIncident: On a visit to a rich friend's estate, Frank recalls how it's been a month since their last visit, and wonders if the maid will still remember them. Joe quips that she probably doesn't ''want to'' remember Frank after scaring her with a skeleton.
* TheSmartGuy: The most intellectual out of the Hardy Boys' regular group of friends.
* TallDarkAndHandsome: Often described as 6" 1' with dark brown hair, and handsome.
* TeenGenius: Frank probably knows more than any teenager should about dealing tactically with terrorists.
* TranquilFury: Chet has had enough of a racist politician's comments about his friends, and tells the burly bodyguard that he should just take his boss and leave the premises. When Frank manages to calm him down, the bodyguard decides to pull a sucker punch that knocked the wind out of poor Chet. Seeing what happened to his friend, Frank calmly challenges the bodyguard to try the same stunt again, this time for someone who's ready. He threw another punch...and Frank easily sends him flying out of the house with a judo throw.

!! Joe Hardy
'''First appearance''': #1 ''The Tower Treasure''

The younger Hardy brother, 17 as of 1974. Blonde-haired and emotional, Joe is hot-headed and prefers taking action over waiting for things to happen; he works on a strong sense of instinct, which usually turns out to be right. The more athletic of the brothers, he is an excellent fighter.

* AnnoyingYoungerSibling: As their characters diverged and got more distinct personalities, he became the much more impulsive, hot-headed and girl-crazy red one to Frank's much more level-headed and intellectual blue one.
* BerserkButton: For the ''Casefiles'' version, it's Iola's death. Or being associated with the Assassins.
** A captive Assassin, a subordinate to Iola's killer, tries to invoke SuicideByCop (since "it's rare to capture an Assassin, alive" seems to be their official slogan) when he tells Joe that he helped to work on the bomb. Joe might have actually killed the guy with his bare hands if not for the agents present.
* BewareTheNiceOnes: He's a good guy at heart, but he tends to lose his temper.
* BigBrotherWorship: in the Undercover Brothers graphic novel series.
* TheBigGuy: Although shorter than Frank, he's often described as stocky and more muscular.
* BornDetective: Raised by former NYPD detective and now current PI father, Fenton.
* BrainsAndBrawn: The brawn half.
* DependingOnTheWriter: Again in the ''Casefiles,'' due to being written by a great many ghostwriters. He would frequently slingshot back and forth between grieving widower and his typical flirty, girl-crazy self.
* DistressedDude: Frequently kidnapped as part of the story, sometimes with Frank, sometimes solo.
* GutFeeling: Relies on these more than logic.
* ILetGwenStacyDie: Once more, Iola.
* ItsPersonal: Whenever dealing with the Assassins, Joe will always remember Iola.
* MadeOfIron: Once took on a terrorist who issued a disabling cut to his hand, and crawled away from a serious car accident with nothing more than a bad case of amnesia.
* TeacherStudentRomance: Case 105: ''Bad Chemistry'', where Joe's new-found interest in Chemistry is the young new teacher. Frank won't stop needling him for this.

!! Fenton Hardy
'''First appearance''': #1 ''The Tower Treasure''

The father of the Hardy clan, Fenton is a retired NYPD detective who runs his own private investigation firm. Although he is proud of his sons' mystery solving skills, he sometimes worries if they've gotten in over their heads; still, he appreciates their assistance on most cases.

* MasterOfDisguise: even his own sons are fooled until Fenton willingly gives himself away.
* PapaWolf: Frequently leads the charge when his boys are missing or in danger.
* RetiredBadass: When he's not being captured, he's been shown to be a competent fighter. Especially true of the ''Casefiles'' version. Even after he's been long retired, he still is frequently brought back in frequently to consult for various organizations, including Scotland Yard.
* StandardFiftiesFather: Played straight in the 1950's and 1960's, naturally. He's since modernized a bit.
* WhatHappenedToTheMouse: Fenton used to have dealings with the Network, which allowed him to pull a few favors when his family was in danger. This connection was rarely, if ever, mentioned again whenever the Hardy Boys have to tackle with the Network in later cases.
* WorkingTheSameCase: the mysteries that the boys are involved with sometimes overlap with his own work.

!! Laura Hardy
'''First appearance''': #1 ''The Tower Treasure''

Frank and Joe's mother, and Fenton's wife. She has very little to do with her husband or sons' investigations and adventures, preferring a quiet home life.

* ExtremeDoormat: She occasionally worries over the dangerous activities of her husband and sons, but is usually dismissed or ignored.
* StayInTheKitchen: By choice; she wasn't the adventurous type and rarely seemed to venture out of the house.

!! Gertrude Hardy
'''First appearance''': #3 ''The Secret of the Old Mill''

Fenton Hardy's sister who lives with the family. Head-strong and stern, she often scolds the Hardy brothers for the dangers they put themselves in, but is secretly proud of them and supports them where she can. In her most recent incarnation, she's known as Trudy.

* MaidenAunt: One of the classic trope examples.
* MamaBear: Just as protective of her nephews as if they were her own sons, and rather more forceful than her sister-in-law.

! Friends and Allies
!! Chet Morton
'''First appearance''': #1 ''The Tower Treasure''

The brothers' oldest and most reliable friend, Chet has appeared in all but a handful of the regular Hardy Boys books. He is often described as fat or chubby and constantly eating. He usually reacts with great enthusiasm when he gets to help in a case, at least until danger becomes a real possibility. In many of the early books, he was always trying new hobbies, which coincidentally always seemed to correspond with the case at hand. He is also Iola's older brother.

He was actually popular enough to have the StratemeyerSyndicate consider giving him his own series in 1965, but it [[WhatCouldHaveBeen never got past the planning stage]].

* TheAllegedCar: an old yellow jalopy he calls ''Queen''. A RunningGag in the earlier books would involve characters would complain about the pain they have to endure when riding it.
* BigEater: Portrayed as constantly eating, even in the more current books.
* FatBestFriend: Ranges across all three types due to the different writers, but mostly between Types B and C. Usually fearful of a situation, but will pull through to help his friends.
* FatIdiot: Played straight in the early volumes. He eventually got smarter (and braver).
* FleetingPassionateHobbies: Commonly in the older books, when the writer needed to introduce some central gimmick (Zodiac fortune-telling, or fencing), Chet would be pick it up as a hobby just in time for the relevant case. The books even commented on it.
* PluckyComicRelief: More often than not, though Chet would get [[ThrowTheDogABone heroic moments here and there]].
* StoutStrength: His build seems to be a boon to him considering he's also a good football player.
* WeightWoe: In some books, he would go on a diet. [[StatusQuoIsGod By the end of the book]], though, his love of food always won out.

!! Allen "Biff" Hooper
'''First appearance''': #1 ''The Tower Treasure''

Appearing in almost as many of the early books as Chet, Biff is the Hardys' super-athletic friend, a lover of sports and physical activity. A keen boxer, his fighting skills almost match Frank and Joe's.

* BigFriendlyDog: in ''Secret Warning'', Biff won a ''Great Dane'' during a contest, which he christens Tivoli, and the Hardys offer to take care of him when the Hoopers are on vacation. An AngryGuardDog [[EpicFail he's not]], but Aunt Gertrude eventually warmed up to the mutt after being convinced Tivoli's heart is in the right place.
* DumbMuscle: In the original volumes, he played more to this. He got smarter as the series went on.
* EmbarrassingFirstName: He doesn't like his actual first name for some reason, and is rarely referred to as anything other than Biff.
* GentleGiant: He's the strongest of the Hardys' friends, but very friendly and generally good-natured.

!! Callie Shaw
'''First appearance''': #1 ''The Tower Treasure''

Frank's steady girlfriend, who remained largely a background character in the main series. In newer volumes, especially the ''Casefiles'' series, she has a much more developed personality and becomes much more independent, sometimes stubbornly insisting that she help the brothers on cases. Her best friend is Iola Morton.

* ActionGirl: She becomes this in the ''Casefiles'' series after her best friend Iola dies.
* DamselOutOfDistress: In the ''Casefiles'', especially volume 10, ''Hostages of Hate''.
* StayInTheKitchen: In the ''Casefiles'', Joe has this attitude towards her at first. Justified, as he had just lost his own girlfriend because of her involvement with the Hardys.

!! Iola Morton
'''First appearance''': #1 ''The Tower Treasure''

Chet's younger sister and Joe's steady girlfriend, also not much more than a cheerful, giggling girlfriend for many of the original books. She acquires a bit more personality and personal interests later down the line.

* GirlNextDoor: Often used to describe her word for word. Oddly, averts all three romance types; she was always more or less Joe's girl.
* KilledOffForReal: In the first volume of the ''Casefiles'' series.
* NeverGotToSayGoodbye: Iola got angry at Joe for ignoring her while he was trying to make a move on another girl that he's just met. Iola goes off to the Hardys' car to let off some steam, and...
* RevengeByProxy: The bomb that killed Iola was actually meant for the Hardys.

!! Tony Prito
'''First appearance''': #1 ''The Tower Treasure''

The son of Italian immigrants, Tony is a close friend of the Hardy Boys, helping out on many cases. Friendly and outgoing, he helped in many early volumes with his own motorboat, the ''Napoli''. He later managed the pizza parlor at the local mall, a post that made him a good connection for watching the comings and goings of the town.

* ChekhovsGunman: He mentions working in his father's construction company in the original series, but in the ''Casefiles'' he has a new job: assistant manager of the fast-food joint Mr. Pizza. Wouldn't you know it, his new job plays an important role for the Hardys' investigation in the ''Dead on Target'' case.
* CoolBoat: Owner of the ''Napoli'', which helped out and rescued the Hardy Boys on a number of occasions.
* GratuitousItalian: Tony and his father used it in the first few books. Quickly phased out.
* NationalStereotypes: Averted, with a few early exceptions. For most of the series, Tony was portrayed as a normal, American teenager. Though he did eventually manage a pizza parlor...

!! Phil Cohen
'''First appearance''': #1 ''The Tower Treasure''

Phil appeared less frequently than Chet, Biff, and Tony, but was another frequent friend of the Hardys'. In the early volumes, he was more artistically talented, but became gifted with electronics and gadgets, eventually evolving into a computer hacker as the times changed. Often introduced in early volumes as the Hardys' "[[ValuesDissonance Jewish friend]]".

* JewishAndNerdy: Phil is the best of the Hardys' friends with technology, and was often noted to be the smartest of them.
* MrFixit: Helped the Hardys fix vehicles on a number of occasions, especially their decked-out black van from the ''Casefiles''.
* NiceJewishBoy: Played straight in the original editions, but dropped in later revisions and books.

!! Sam Radley
Fenton Hardy's top investigator. Helped the Hardy Boys on a number of cases, and got into his fair share of trouble as well.

* PrivateDetective: Working for Fenton.
* NumberTwo: Seems to be this, especially when Fenton is unavailable.

!! Vanessa Bender
'''First appearance''': ''Casefiles'' #69 ''Mayhem in Motion''

Appearing only in the ''Casefiles'' series, she is a client who the Hardy Boys helped. At the end of the case, she and Joe began dating, and by the time the series was canceled, they were going steady.

! The Authorities
!! Chief Ezra Collig

Chief of Police of Bayport, Collig originally served as more of a foil in the early books, often embarrassed by being upstaged by the Hardys' amateur antics. Over the years, he has come to accept their help and put his full trust in them, although he wishes they would pick a safer hobby.

* FriendOnTheForce: Eventually becomes this for the Hardy Boys.
* ReasonableAuthorityFigure: Usually listens to the Hardy Boys warnings and ideas, but is frequently unable to act on anything until they come up with proof. On occasion, he actually asks them for help.

!! Con Riley
A veteran officer of the Bayport Police, Riley, like many adults, underestimated the Hardy brother's severely, and in the early books was often aggravated by their and their friends' antics. He has evolved into a much more competent, patient character over the years and has come to regard the brothers as a reliable ally in solving crimes around Bayport.

* AscendedExtra: Was little more than a comic relief character. Eventually became their most frequently used contact with the police next to Chief Collig.
* FriendOnTheForce: Becomes this for the Hardys, particularly in the ''Casefiles'' series.

!! Oscar Smuff
Appeared in many of the early volumes. A stuffed-shirt who viewed himself as brilliant, a personality that often caused him to be the target of pranks by the Hardys and their friends, all of which ended up making him look like a complete fool. Eventually, his buffoonery got him busted down to patrolman, and he was eventually let go from the force altogether.

* ButtMonkey: Existed purely to be the butt of the Hardy Boys and friends' jokes, or perpetually tried to arrest the wrong person.

!! The Gray Man/Arthur Gray
'''First appearance''': ''Casefiles'' #1 ''Dead On Target''

The Gray Man, another ''Casefiles''-only character, works for the secret international organization "The Network", often working with cases involving terrorism. He is completely unassuming and blends in perfectly with the crowd, but is extremely observant and good with both fists and guns. He resented their interference in their first case together, which created issues between him and the Hardy brothers on several encounters. He still considers them little more than talented amateurs, but has gradually come to respect their skills and occasionally call on their help.

* AloofAlly: Knows the Hardys can get results, but reluctant to put them into danger given their relative inexperience.
* CloakAndDagger: The Gray Man (and the Network) seems to be very much in this vein.
* OvertOperative: Averted; he's average, boring and nondescript in appearance.
* PetTheDog: Mr. Gray does come to respect the Hardy Boys. Eventually.
** In Case 3: ''Cult of Crime'', since the boys have been charged with murder, he mentions that he cannot get the Network involved when they try to contact him for help. He does, however, accept their request to pick up the Hardys' black van (which had to be left behind when on the run), and ends the transmission wishing Frank and Joe good luck.
* RidiculouslyAverageGuy: In almost every appearance, he's described as the most ridiculously bland human being you could imagine.
* TheSpymaster: Becomes the Hardys' "handler" when he and the Network need their help.
* WorfHadTheFlu: hunting down the Assassin that tried to kill the Hardy Boys is his case. However, he couldn't see it to the end (which he immediately lampshades upon contacting Frank and Joe when the crisis is over) because [[spoiler:he got caught in a explosive booby trap that puts him in a coma (had Frank not pushed him away, it could've been worse) for roughly half of the book.]]
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