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.)
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.
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.
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 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.
Tykebomb: Like Ian, she was trained by her mother to become a clue-hunter and dangerous Lucian agent.
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.
"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.
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.
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
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.
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.
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.
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.
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.
Characters From the Sequel Series, Cahills vs. Vespers
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.