* CanonSue: Arguably, Mistaya. Aside from being [[EverythingsBetterWithPrincesses the princess]], doted on by her parents, the instrument of her grandfather burying the hatchet with Ben, tutored (and adored) by Questor and Abernathy, and loved and fiercely defended by Strabo, she is a child of Landover, Earth, ''and'' the fairy mists and thus has incredible magical power which manifests from birth. She ages rapidly, as well as having a far more adult mind before her time, and her magic which has already proven to be on a par with Nightshade's is only getting stronger with age. The only things that keep her from being a true MarySue are the fact there are others remaining with equally powerful magic (Ben, Nightshade, Strabo) to keep the narrative balanced, that as shown by ''Princess'' even she doesn't know everything and often needs help, and the fact that in the end she still manages to be rather likable.
* MoralEventHorizon: Using the Darkling to compel Willow's mother to dance for him came very close to this for the River Master. Luckily he realized in time and let her go.
* NightmareFuel: An Ardsheal is an [[LightningBruiser ungodly-fast, super-strong]], [[MadeOfIron nigh-indestructable]] humanoid elemental that is virtually invisible when it wants to be. In ''Witches Brew,'' the River Master summons one as a bodyguard for Willow and Ben. Then [[spoiler:when Nightshade gets a hold of it]] we find out exactly why Willow is terrified of it.
** What Mistaya discovers in the stacks of Libiris might qualify too, particularly when it is revealed the dark, black-hole like manifestation is created by [[spoiler:Libiris itself, crying out for help and [[DoesThisRemindYouOfAnything in reaction to being tarnished as it loses its books to Abaddon while its walls are penetrated]]]].
* TrueNeutral: At first the fairies seem to hew to this trope, but eventually it is revealed that they will take sides, if it means preventing [[TheEndOfTheWorldAsWeKnowIt the destruction of Landover]] or maintaining the BalanceBetweenGoodAndEvil (or Law and Chaos). And every time they do choose to act, it is always the key to salvation: in book 1 their gift of Io Dust is what allows Ben to get rid of Nightshade and stop Strabo's depredations; in book 2 Dirk's assistance to Ben and the gradual alteration of Willow's dreams allows them to stop Meeks and save the unicorns; in book 4 it is the fairies allowing Willow and Ben to encounter each other in the mists, circumventing the Tangle Box, that helps him recover his memories of his true self (and them sending Dirk to Willow allows her to gather all the soils she needs and give birth to Mistaya); and in the most recent book they send Dirk again, this time to Mistaya to help her save Libiris (and Landover) from Crabbit and the demons.