* AlternativeCharacterInterpretation: Is Alistair Oh an old man who genuinely cares for Amy and Dan and feels regret for [[spoiler: leaving them for dead time and time again and being there when Isabel set the fire that killed their parents]], or a backstabbing enemy who feels no remorse for his betrayals?
** Ian Kabra tends to be subject to this for ''several'' reasons, but since his [[spoiler: [[ConflictingLoyalty going rogue]] in ''The Dead of Night'']], the question of his true nature is definitely up for debate. [[spoiler: However, it turns out that his departure had nothing to do with any form of betrayal (except his mother's) and that TheMole was actually Sinead Starling.]]
* AngelDevilShipping: The [[FanPreferredCouple very popular]] [[ShrinkingViolet Amy]]/[[HandsomeDevil Ian]] pairing.
* BrokenBase: The [[AllThereInTheManual trading cards and website,]] especially the cards. Why? In the opinion of many fans, the pictures of the characters on the trading cards do not live up to the characters' descriptions in the books or the way that readers imagined the characters to look like, or else simply do not look appealing (a common complaint is that Ian and Evan look too much like Music/JustinBieber). The rest of the fans either like the pictures on the trading cards, or just like the trading cards and couldn't care less about what the pictures look like.
** When the official message boards were made canon in ''The Dead of Night'', some fans blew a gasket. Others eagerly played along with Evan's posts on the message boards. It's hard to find someone without a clearly defined opinion on this development...
* DieForOurShip: Fans of [[FanPreferredCouple Amy/Ian]] have been known to behave extremely aggressively towards any and all opposing pairings, bashing [[LikeBrotherAndSister Hamilton]], [[GirlOfTheWeek Kurt]], and [[SecondLove Ev]][[OfficialCouple an]], complete with a plethora of RevengeFic for the latter two.
** Shippers of [[ShipsThatPassInTheNight Hamilton/Sinead and Jonah/Sinead]] do not often get along, for this very reason.
* EnsembleDarkHorse: The Kabra siblings, especially Ian, are quite popular within the fandom.
* FanDumb: [[https://www.fanfiction.net/r/5773510/ Most of the entire review section for this fanfic]], where there is gigantic amounts of DieForOurShip.
* {{Fanon}}: And a good amount of it, too.
** It is difficult to find a fan who does not believe that the [[DieForOurShip infamous]] [[GirlOfTheWeek Kurt]], who briefly appeared in one book and was never seen again, was an undercover Vesper.
** A friend of Amy and Dan's Uncle Shep was mentioned ''once'' in the first series. But due to his name ([[spoiler:Gregory Tolliver]]) and the revelation that [[spoiler: Arthur Trent, Amy and Dan's father and Shep's cousin, was a Vesper]], people not only believe that this friend is probably a Vesper, but that Evan Tolliver is related to Vespers as well (if not a full-blown undercover agent). This fandom sure does love its EpilepticTrees...
** Since the revelation of Isabel Kabra's maiden name (Vesper-Hollingsworth) and of the fact that Arthur Trent [[spoiler: is a Vesper]], a great deal of fanon involving them has sprung up. It's taken for granted that they interacted with each other when younger, and many fans believe that they were romantically involved, though there is no canon basis for this. (Although the fact that [[spoiler: Isabel knew about the "moon face" thing that Arthur and Dan used to do, enough to use it to manipulate Dan with the "AJT" texts,]] may imply that they were close.)
** Quite a few fans are absolutely convinced that Amy's full name is Amy Hope Cahill, seeing as her brother's full name is Daniel ''Arthur'' Cahill.
** It's generally accepted that Amy and Dan got their green eyes from their mother, and although {{Canon}} gives no evidence supporting this theory.
** It's also generally accepted that [[TheSmartGuy the Starling triplets]] suffer from some form of ParentalNeglect, ParentalAbandonment, or AbusiveParents. The only canon support for this theory is that their parents are never seen or mentioned.
** Speaking of the Starlings, it's become a popular idea that, if [[TeenGenius Sinead]] and [[JiveTurkey Jonah]] are ever in close proximity, epic SnarkToSnarkCombat will ensue.
* FanonDiscontinuity: ''Many'' fans would like to pretend that ''Day of Doom'' never happened, [[OutOfCharacterMoment for]] [[RomanticPlotTumor multiple]] [[KilledOffForReal reasons]].
* FoeYay: Seeing as most of the main characters are adolescents caught up in centuries-old feuds, this is eveywhere. Mostly between Amy and Ian, [[spoiler:especially in the third book]], but it can be seen between other clue hunters, too.
** I thought FoeYay was supposed to be ''subtext''. Amy and Ian are practically the OfficialCouple... at least in the first series.
* FridgeBrilliance: In the first series, Amy and Dan were often shown to have [[TwinTelepathy sibling telepathy]], but that has all but disappeared in the SequelSeries. It is because, as Dan grows darker and more emotionally distant from his sister, he and Amy can't understand each other as well as they used to.
* FunnyAneurysmMoment: [[spoiler: Ian's famous "Shoot to kill?" line in ''The Medusa Plot'' is suddenly not so funny anymore after Evan is ''actually'' shot and killed at the end of the SequelSeries. After he and Ian seemed to have been starting to develop a friendship, no less.]]
* GodModeSue: What you will become if you drink the master serum.
** [[spoiler: And Isabel did.]]
* HarsherInHindsight: [[spoiler: The ending of ''The Sword Thief'', where Alistair fakes his death to escape and later in the first series, when he pretends to die of heart failure to trick his EvilUncle, are not nearly as amusing after reading ''Trust No One'', where he is KilledOffForReal as a result of a seizure.]]
** [[spoiler: Ian's thoughts when his sister is kidnapped aren't exactly uplifting to begin with, but they become even more painful after she dies, along with their estranged mother.]]
---> "It was funny... [Ian] didn't even like [[AnnoyingYoungerSibling Natalie]]. But now that Mum had disowned them and Father was out of the picture, Natalie was his whole family."
* HoYay:
** Fiske and [=McIntyre=].
** Dan and Atticus, and ''how''.
** Jonah and Hamilton.
* JerkassWoobie: Ian and Natalie become this after they undergo a lot of CharacterDevelopment. And their [[EvilMatriarch scary mother]] shows up... [[spoiler: and eventually tries to kill them, along with the other Cahills]].
** And by the end of the ''Cahills vs. Vespers'' series, Ian has it even worse than before. [[spoiler: His sister, the only person he really had left, is killed in ''Day of Doom.'' Their mother, Isabel, finally shows a shred of humanity upon learning of Natalie's death--but in her rage, she drinks the master serum and ends up dead, as well (though not without a glorious DyingMomentOfAwesome). His father is still in hiding and seems to want nothing to do with him. And that's probably just the tip of the iceberg, considering all the issues that being raised by Isabel left him with, as well as the fact that not everyone in the Cahill family completely trusts him yet.]]
** [[spoiler: Sinead Starling]], post-FaceHeelTurn.
* LauncherOfAThousandShips: In canon itself, [[BadassBookworm Amy Cahill]]--[[HandsomeDevil Ian Kabra]], [[GirlOfTheWeek Kurt]], [[NiceGuy Evan Tolliver]], [[JerkWithAHeartOfGold Jake Rosenbloom]] (to say nothing of {{Fanon}}, which has shipped her with [[TheBigGuy Hamilton Holt]], [[WellDoneSonGuy Jonah Wizard]], and [[TheSmartGuy Ned and Ted Starling]], among others). Then, in fanon, we have [[TeenGenius Sinead Starling]]. The most popular Sinead ships are [[BrainsAndBrawn Sinead/Hamilton]] and [[SnarkToSnarkCombat Sinead/Jonah]], but she has also been shipped with [[BlondGuysAreEvil Casper Wyoming]] and occasionally [[PairTheSmartOnes Ian]], and even fanfiction shipping her with [[AnnoyingYoungerSibling Dan Cahill]], [[BrotherSisterIncest Ned]], or [[LesYay Amy]] have made appearances.
** [[KidHero Dan Cahill]] also qualifies. He's been shipped with [[CuteBruiser Reagan and Madison Holt]], [[LittleBoySeeksBigGirl Sinead Starling]] (as mentioned above), [[HoYay Atticus Rosenbloom]], and more than one MarySue original character. Though the likeliest candidate is [[RichBitch Natalie Kabra]]; the two are shipped together only slightly less frequently in the fandom than their older siblings, [[FanPreferredCouple Amy and Ian]]. (And this is all while Dan remains a ChasteHero in canon.)
* MindGameShip: [[OppositesAttract Amian]], in canon.
* RomanticPlotTumor: In ''Day of Doom'', especially with the by now infamous [[spoiler: Amy and Jake LoveConfession in the closet]] scene. Even many of the pairings' supporters found its portrayal in that book to be cheesy, OOC, and ill-timed.
* RonTheDeathEater: Quite frequently for [[DieForOurShip Evan Tolliver and Kurt]]. The most commonly found demonization is making them turn out to be undercover Vespers, but fanfiction involving Evan suddenly dumping Amy is also becoming common.
** Luke Cahill is sometimes treated this way, for no apparent reason other than he liked to sneak around more than his siblings. He did also become somewhat of a JerkAss in his later years, but he was never ''evil''.
* {{Sequelitis}}: The SequelSeries, ''Cahills vs. Vespers''.
* ShipMates: [[FanPreferredCouple Amy/Ian]] fans have a tendency to also support [[OppositesAttract Dan/Natalie]] (which would lead to a DoubleInLawMarriage). Said fans also generally ship either [[BrainsAndBrawn Hamilton/Sinead]] [[ShipToShipCombat or]] [[SnarkToSnarkCombat Jonah/Sinead]].
* TheWoobie: Dan and Amy.
** At least towards the end of the series, [[spoiler: Ian and Natalie.]] In the beginning, they were quite cruel. But after we meet [[spoiler: their mother...]]
* ToyShip: [[FanPreferredCouple Dan/Natalie]], Dan/Reagan, Dan/Madison, [[HoYay Dan/Atticus]]...