* CompleteMonster:
** [[CatsAreMean Jupiter]] is a [[EvilSorcerer powerful sorcerer]] and the BigBad for the original trilogy. [[FromNobodyToNightmare Born]] as a deformed kitten named [[spoiler:Leech]], he grew envious of his older brother and hated him for possessing magical powers he longed to have. After he [[spoiler:lets his brother die in a fire, he gains control of his body]] and later becomes a [[GodOfEvil malicious god-like entity]]. With hundreds of rats at his disposal, Jupiter forced his army to dig underground so he could unleash the Plague onto the world, and allowed his rats to slaughter dozens of innocents. When his plans to unleash the Plague fail and his spirit is trapped in limbo, he cajoles [[FortuneTeller Madame Akkikuyu]] to try and resurrect him, and even convinces her to bring a corn dolly to life, which later went on and [[WouldHurtAChild murdered two Fennywolder mouselets]]. Once his spirit was released, Jupiter quickly took control of a power station and started to [[KillItWithIce freeze the city]]. After gaining more power, he brainwashes [[TheDragon Morgan]] into bringing his horde of rats to the station so he can kill them all and use their spirits as his new soldiers. He then attacks [[TheHero Audrey Brown]]'s friends and family, killing several of them all while he attempts to [[OmnicidalManiac destroy the sun and start a new ice age]]. A savage, bloodthirsty cat [[AGodAmI who demanded to be worshipped by all]], Jupiter only cared about [[spoiler:killing his brother]] and destroying the world, [[TheUnfettered no matter how many innocent lives were lost or how many loved ones he chose to neglect]].
** ''Deptford Histories'':
*** ''The Oaken Throne'':
*** [[BigBad Lady Morwenna]] is the [[EvilChancellor traitorous handmaiden]] for the Starwife and the HighPriestess for the [[ReligionOfEvil Hobbers]]. After years of planning, Morwenna [[BodyguardBetrayal betrays and kills the Starwife]] before ordering Rohgar and his legion of bats to destroy Greenreach; anyone who survived was [[EatenAlive fed to her pet toads]]. Seeking the [[AmuletOfConcentratedAwesome Silver Acorn]], Morwenna also has the bats bomb [[TheHero Ysabelle]]'s hometown of Coll Regalis, before ordering the Hobbers to hunt her and her friends down and kill anyone who gets in their way. After the Hobbers and bats fail, Morwenna finds Ysabelle herself [[BitchInSheepsClothing while posing as a good Samaritan]], and later steals the Acorn from her and tries to feed her to her toads. With the Acorn in her possession, she wastes no time [[ChronicBackstabbingDisorder betraying and killing Rohgar]], and once [[GodOfEvil Hobb]] is resurrected, she immediately tries to use him to kill Ysabelle. A cunning squirrel who enjoyed [[LovesTheSoundOfScreaming hearing her victims scream]], Morwenna stands out as being one of the few villains in the series who resurrected a demon and nearly used him to TakeOverTheWorld.
*** [[spoiler:Wendel Maculatum]] is a [[BitchInSheepsClothing traitor in disguise]] and the HighPriest for the Hobbers. Posing as [[spoiler:a comical jester]], he traveled alongside Ysabelle and her group in hopes of acquiring the Silver Acorn. After separating from her group, he and his Hobbers kidnap or kill her and her friends before he uses the Acorn and the blood of three dead squirrels to summon Hobb. He and his Hobbers constantly stalk and taunt Ysabelle's group as they travel through the woods, even going as far as [[DecapitationPresentation throwing a mouse's head at them]] just to scare them. When his true identity is discovered by [[spoiler:Tysle Symkin]], the former immediately murders the latter and flays his corpse, leaving it for Ysabelle's friends to find. His Hobbers later attack and destroy a stronghold full of warriors; while fleeing with Ysabelle's group, he finally reveals his identity and [[EvilGloating gloats how Ysabelle and her friends are doomed]], and nonchalantly taunts [[spoiler:Giraldus]] over [[spoiler:Tysle's]] death. [[spoiler:Even after Wendel dies, he reappears as a ghost just to kill Vesper out of spite and to fulfill a curse he placed on him earlier]].
*** ''Thomas'': The [[NoNameGiven unnamed]] HighPriest of Sarpedon is [[BigBad the leader]] of [[ReligionOfEvil the Scale]], which longs to resurrect the [[SnakesAreSinister serpent]] god [[GodOfEvil Sarpedon]], also known as [[IHaveManyNames Suruth Scarophion and Gorscarrigern]]. Seeking nine fragments of a [[EggMacGuffin jade egg]], the high priest and his vast Scale army scourged the world and killed anyone in their path. After running into the ''Calliope'', he capsizes the ship and kills hundreds onboard before heading to the Temple of the Twelve Maidens and massacring everyone in and around the temple, [[DeadGuyOnDisplay leaving their defiled corpses to be found later]]. After acquiring the seventh fragment, the high priest and his army head to Hara, where he attacks the city and slaughters thousands in hopes of finding another fragment. Once the high priest steals the last two fragments from Thomas, Woodget, and Simoon, he takes them all to the Black Temple so he can resurrect Sarpedon and offer them to the god as a sacrifice. When Thomas and his friends greatly damage the high priest's army, he chases after Thomas's crew using a statue possessed by Sarpedon's spirit, longing to kill them out of spite.
* JerkassWoobie: Isaac Nettle. You can't help but feel sorry for him as [[spoiler:he grieves over the death of his son, even though he [[AbusiveParents was abusive]] towards him. It was heartbreaking when he found the owl pellet containing Jenkin's mousebrass.]]
* UncannyValley: The illustrations, to some extent, but considering the book's content this is likely intentional.
* WhatDoYouMeanItsForKids: While these books feature cute animals, many of them are subjected to violent, absolutely horrific deaths such as being skinned alive, decapitated, or having their body slowly eaten away by a deadly poison.
* TheWoobie: There are plenty of these, with Audrey and Ysabelle being among the most notable examples.