Annoying Younger Sibling: Starts out as this. In the end, he still gets on Amy's nerves fairly frequently, but they have been through a great deal together and have realized that their strengths and weaknesses balance each other out.
Chaste Hero: Justified, as he's only 11-13 in the series. (Also, he's probably seen from his sister's love life that romance only complicates things, and his life is complicated enough as it is—what with escaping homicidal enemies, trying to save the world, being an awesome ninja, and all that.)
Class Clown: Apparently, he was one prior to the Hunt.
Collector of the Strange: While Dan collects some normal things, such as baseball cards, he's also been known to collect odder things, like tombstone rubbings.
Hero with Bad Publicity: Wanted by Interpol as of Cahills vs. Vespers. However, he and Amy are only doing what they have to, cooperating with the Vespers to ensure the safety of the hostages.
Heroic BSOD: At the end of Storm Warning, after Lester's death.
Hypocritical Heartwarming: Will get angry whenever anyone else insults/hurts his sister. It doesn't change the face that he also regularly makes fun of her, but it's sweet nonetheless.
Pursuing Parental Perils: He and Amy are searching for the 39 Clues, a quest that resulted in their parents being killed in a fire by their clue-hunting rivals.
Rags to Riches: Everyone who passed through the gauntlet gained two million dollars, twice the amount offered in Grace's will.
Twin Telepathy: Although they have a three-year age difference, he and Amy share a bond like this.
Upbringing Makes the Hero: His and Amy's naivete in regards to the clue-hunting world is what gives them an edge against their competitors.
Amy Cahill
Action Girl: Eventually becomes one, as exhibited nicely at the start of The Medusa Plot when she kicks a gun out of a man's hand after her and Dan's school bus was attacked.
Action Survivor: Both she and Dan, until the start of Series Two.
Naïve Newcomer: At the beginning of the first series; by the Sequel Series, she's the de facto head of the Madrigals.
Official Couple: At the start of Cahills vs. Vespers, she's dating Evan Tolliver, However, at the end of that series, he is killed in the fight against the Vespers, and she starts going out with Jake.
Screw the Rules, I'm Doing What's Right: She and Dan are forced to make the "abide by the law or do the right thing" decision a lot, and they inevitably do this.
She Cleans Up Nicely: Judging by Ian's uncharacteristic reaction to her wearing a fancy dress in Rapid Fire: Fireworks.
Unrequited Love Switcheroo: Ian fell for her just as she was beginning to (allegedly) get over her crush on him—although Crushed later showed that she wasn't quite as "over him" as she believed herself to be.
Upbringing Makes the Hero: Her and Dan's lack of knowledge about the clues gives them a unique perspective on the Hunt, as well as keeps them from doing such reprehensible things as the other teams do to get clues.
Chekhov's Skill: Has many talents that come in handy during the clue hunt.
Contrived Coincidence: Nellie's learned so many things that help the Cahills on their quest.
Although near the end of the series it's revealed that Grace knew she'd end up being their nanny one day, so she paid for plenty of Nellie's learning (such as being trilingual and knowing how to fly a plane).
Cool Big Sis: To Amy and Dan. She becomes an actual "big sis" to them, or at least family, when she becomes one of their legal guardians at the end of the first series.
Dreadful Musician: Apparently, Nellie's musical talent... leaves quite a bit to be desired.
Less Embarrassing Term: She says she signed on as an au pair and not a nanny, despite the fact that the former is basically just Gratuitous French for the latter.
Mama Bear: Though her relationship with Amy and Dan becomes more reminiscent of a sibling bond, this still applies.
Trademark Favorite Food: He loves red snapper—so much that, in One False Note, he refuses to eat anything else. (However, in The Medusa Plot, it's shown that he has also developed a taste for caviar.)
Enemies in the Clue Hunt
Ian Kabra
Ian is introduced as Natalie's handsome, scheming older brother, befitting his status as a high-ranking Lucian. Like his sister, he joins the clue hunt on orders from his mother, whom he follows blindly. In The Sword Thief, he plays on Amy's crush on him and uses her and Dan to get the Clue and leaves them for dead in a cave. This leaves him with a nagging feeling of guilt which culminates in his and Natalie's abandoning the Kabra family at the end of the series. As of the end of Cahills vs. Vespers, he is completely on his own, as both his mother and sister are dead, and his father is still in hiding.
Betty and Veronica: The Veronica to Amy's Archie in terms of personality, but the Betty in that he seems to have known Amy for longer than Evan or Jake have. (He's said so himself: they have a "considerable history.")
Break the Haughty: After his mother's betrayal, although he didn't take it as badly as Natalie.
And then, it gets worse. In Cahills vs. Vespers, his sister is held captive by Vespers, and when they finally reunite, she is killed by the Vespers' Doomsday machine—actually bringing Ian to tears. Soon afterward, his mother dies, as well.
Chick Magnet: He is said to have numerous girls fawning over him, though the only named character who has been shown to harbor an interest in him was Amy.
Conflicting Loyalty: Faces a great deal of it in the latter half of the original series.
Insecure Love Interest: The reason why he decided not to confess his feelings for Amy in Crushed was due to the fact that, after letting his mother go, he felt that he didn't deserve Amy.
Smug Snake: Or, one might say, a cocky Cobra. He gets a little better after his Heel Face Turn, although he definitely still has his moments of arrogance.
Stalker with a Crush: Operation Trinity shows him watching Amy come home from school via a surveillance camera (although, technically, the crush didn't really manifest until later).
The Proud Elite: He's dark, described as very good-looking, and is extremely full of himself. He starts off pretty evil, but turns out to have hidden morals.
The Rival: He and Natalie are a Brother-Sister Team like Amy and Dan, but with more money and resources. Ian is Amy's age, and Natalie is Dan's.
Too Clever by Half: Is highly intelligent (he can beat a supercomputer at chess) but, as his sister notes, is somewhat lacking in common sense, which can lead him to make mistakes.
Tykebomb: Like Ian, she was trained by her mother to become a clue-hunter and dangerous Lucian agent.
Alistair Oh
A South Korean businessman hailing from Seoul, Alistair made his fortune from selling the microwavable burrito. But by the time of the series his money is running out and he swaps the million dollars for a hint to the first clue. He is an Ekaterina. Throughout the series often offers to help Amy and Dan, but they are openly distrustful of him: because not only does he seem fond of double-crossing them, he was there on the night of the fire that killed their parents. In Trust No One, he is Killed Off for Real.
Never Found the Body: The Sword Thief: While escaping a cave in Korea. Amy and Dan seem genuinely grieved by his death - until they find his glove and realize that he must have left them.
"You are afraid of everything except what you should fear."
A Lucian ex-KGB agent who agrees to trade the one million dollars for a hint to the first clue at Grace's funeral. Throughout the series, Irina works behind the scenes, following Amy and Dan and threatening people with her poisons. By the time the sixth book rolls around, however, it's revealed that there's way more to her than we originally thought. Then, she dies.
A Death in the Limelight: In Too Deep gives her character a lot of development. She dies at the end of the book.
Action Girl: A grown-up version. She was a KGB agent, after all.
Rogue Agent: So she claims. In actuality, she works for Isabel. Look where that got her.
Russian Girl Suffers Most: She lost her son, was treated as a servant by Isabel, and finally dies in a fire when Isabel sees it a bother to rescue her.
International hip-hop sensation and teen idol, Jonah fits very nicely into the family's Janus branch. His mother is the famous Cora Wizard, who set up his career. Jonah joins the clue hunt to please her because she wants the master serum for herself. At the beginning he is only concerned about his public image, but after he befriends Dan in The Emperor's Code this begins to change. Because of his concern of what other people think of him, he doesn't let anyone know about his secret love of Shakespeare's works.
A Tomas family consisting of Eisenhower Holt, his wife Mary-Todd, and their children Hamilton, Madison, and Reagan, the Holts excel at everything athletic but are not the brightest crayons in the box. Out of the five of them, Hamilton is the most sympathetic, and is the one who forms alliances with Amy and Dan. He was the one who went into the gauntlet, and as such the rest of the family was taken hostage by Isabel Kabra.
Big Brother Instinct: Hamilton seems to show a bit toward Dan and Amy later on in the first series. Once, he protectively refers to Dan as "my little buddy."
Genki Girl: Day of Doom compares Reagan to the Energizer Bunny, as she is constantly keeping the other hostages active and helping them keep their strength up.
Genius Bruiser: Hamilton has a talent with computers and did well on his SATs.
Genius thirteen-year-old triplets who turn down their three million dollars in favor of the Clue Hunt. All three are Ekaterina. They don't stick around, however — in the very first book they are severely injured and hospitalized by a blast set up by the Holts for Amy and Dan. They don't appear again until Book 10, wherein it's shown that Ted was blinded by the blast and can only see light and dark, whereas Ned frequently has migraines that interrupt his ability to think. Out of the three it's Sinead who we get to see the most of because she was the least injured. In the Cahills vs. Vespers Mission 4, Sinead is revealed to be working for the Vespers: as Vesper Three, The Mole, in fact. However, she makes a Heel Face Turn in Day of Doom.
Action Girl: Sinead. See the end of The Medusa Plot for further details.
Disability Superpower: Hinted at. They decline the offer of the serum even though it could cure their injuries because they want to develop something that would help other people suffering from the same thing.
Well-Intentioned Extremist: Sinead is working with the Vespers in hopes of being able to cure her brothers. When she realizes that the Vespers will not be able to help her, she makes a Heel Face Turn and rejoins the Cahills.
Other Major Characters
Grace Cahill
Beatrice's younger sister, Fiske's older sister, Hope's mother, and Amy and Dan's grandmother, she was one of the last genuinely nice Cahills. After she dies of sickness she chooses some of her relatives, the ones most likely to succeed in the clue hunt and by extension the gauntlet, and offers either one million dollars or a hint to the first clue to them. Amy and Dan really were her favorites, and they spent almost every weekend with her. She was a Madrigal.
Action Girl: Shown to be this when she was younger.
Amy and Dan's parents. They died in a fire set by Isabel Kabra (accompanied by Alistair, Irina Spasky, Cora Wizard, and Eisenhower Holt and his wife) after they refused to share information and were avid Madrigal clue hunters. Arthur was the one who got Eisenhower expelled from West Point Military Academy. As of Cahills vs. Vespers, it is hinted that Arthur may be alive, but it turns out that he's dead after all.
Deceased Parents Are The Best: Played with a bit. Though Amy and Dan initially remember their parents nothing but fondly, once they learn that they were Madrigals, their feelings are briefly called into check. In the Sequel Series, something similar happens when the siblings find out that Arthur was a Vesper... and believe that he may not have died in that fire after all.
Defecting For Love: When Arthur met Hope Cahill, he was a Vesper. He stopped his Cahill-hating ways once he married her.
Not Quite Dead: Arthur, as of The Dead of Night—at least, so Dan is led to believe by Isabel Kabra.
The Man In Black/ Fiske Cahill
Initially, a mysterious character who seems to be following the Cahill siblings as they hunt for clues. He turns out to be Beatrice and Grace's younger brother, Amy and Dan's great-uncle.
Ian and Natalie's mother, a Lucian whose elegant and superficial exterior conceals a great deal of wickedness. She is the Big Bad of the first series. In Cahills vs. Vespers, she is revealed to be Vesper Two, but after the death of her daughter, she consumes the master serum and kills innumerable Vespers and even Damien Vesper himself, though she dies in the process.
Abusive Parents: And how. Trying to kill her own children, for one thing.
Dying Moment of Awesome: Kills innumerable Vesper mooks effortlessly and manages to take out Vesper One himself after consuming the master serum, dying as she destroys the Doomsday machine that killed her daughter Natalie.
The Fashionista: She can have you murdered and look good doing it.
“Well Done Son” Guy: Both of her children seek to gain her approval (especially Natalie).
With Great Power Comes Great Insanity: After she drinks the Lucian part of the master serum (giving her ultimate intelligence and strategic skills) it's hinted she finally snaps.
Cool Uncle: Amy and Dan call him "uncle," even though he was actually their father's cousin. (He and Arthur grew up thinking of each other like brothers.)
Grace's and Fiske's older sister, an eccentric old woman who could care less about the Cahill family history. It's revealed in the "Black Book of Buried Secrets" that this is only so because she couldn't handle the pressure put on by her sister and brother, who excelled in all the tests.
Jerkass: Is beyond nasty to her great-neice and nephew.
Maiden Aunt: She would fit the trope if Amy and Dan were remotely fond of her. Instead, she is merely a...
Resentful Guardian: She clearly resents being saddled with her sister's grandchildren. Amy and Dan feel the same about her and don't mind being taken care of primarily by a series of au pairs.
The father of Ian and Natalie and the husband of Isabel. Thus far, he has not appeared in the series in the flesh.
Abusive Parents: At least emotionally abusive, if Ian is to be believed.
Parental Neglect: After his wife was put in prison, he fled to Brazil, deserting his children.
“Well Done Son” Guy: Ian craves his approval, according to The Black Circle.
Characters From the Sequel Series, Cahills vs. Vespers
Evan Tolliver
Introduced in Vespers Rising as a boy who goes to school with Amy and Dan. In The Medusa Plot, he has been dating Amy for several months. After the Cahills fend off an attack by the Vespers in full view of Evan, he becomes understandably confused, and gets even more confused and upset when Amy cuts off contact with him in fear of him finding out about her family history. Eventually, however, Evan does find out about the Cahill story, as a result of his immediately being able to identify the obscure type of cell phone that Vesper One is using. Also because of this, Evan is invited to join Ian and Sinead at the Cahill Command Center, and helps them do research to aid Amy and Dan in their missions around the world. Evan is still helping the Cahills in A King's Ransom and The Dead of Night, and is becoming increasingly worried about Amy. In Day Of Doom, he is shot and Killed Off for Real.
Adorkable: At least to his girlfriend, who thinks of him as being this.
Dying Moment of Awesome: Although previously a Non-Action Guy, he helps the Cahills in the fight against the Vespers in Day of Doom and seems to be fairly good at it. It is during this fight that he is fatally wounded by a gunshot.
Stalker with a Crush: Shows shades of this in The Medusa Plot after Amy cuts off contact with him due to problems with the Vespers, going so far as to sneak onto the grounds of Grace's mansion to find out what's going on. This behavior stops once Ian explains the Cahill-Vesper conflict to him.
In Day of Doom, he does end up taking a bullet, although not for any other character in particular.
Understanding Boyfriend: Not only is he far more understanding than many people would be, but he is also very helpful to the cause.
Casper Wyoming
A character, not the place. A Vesper who was named after a city where his parents robbed a bank. He's proud of that. He was formerly Vesper Six, until his twin sister Cheyenne took his place.
Siblings in Crime: He and Cheyenne—and seeing as their parents named them after bank robberies they committed, it apparently runs in the family.
Theme Twin Naming: Both he and Cheyenne are named after their parents' bank robberies in (alliterating) cities in Wyoming.
Cheyenne Wyoming
Like her twin, Casper, Cheyenne is a character not to be confused with the city. She is pretty dangerous, especially when she's working alongside her brother. She is Vesper Six.
Smooch of Victory: Amy gives him a jubilant kiss on the cheek in The Dead of Night when he makes a big discovery, leading a stunned Jake to utter the infamous "Wore yelcome."
Third Option Love Interest: For Amy Cahill. She chooses him in Day of Doom, though it remains to be seen whether this relationship will last in the third series or not.
The parents of Luke, Katherine, Thomas, and Jane Cahill, as well as Madeleine. Gideon invented the master serum which is made by combining the 39 ingredients revealed by following the eponymous Clues.
Luke Cahill
The oldest Cahill sibling, and the founder of the Lucian branch.
Badass Cape: Wears one later in life, when he works as a royal advisor. It adds to his menacing appearance.
Green Eyes: He apparently inherited them from his mother Olivia.
Katherine Cahill
The second-oldest Cahill sibling, who founded the Ekaterina branch.
