Ambiguous Disorder: Lizzie has strong skills with puzzles, riddles, and numbers, has frequent panic attacks, has stunted social skills, only has one friend who is just as odd as she is, and doesn't have much physical prowess. Ripred immediately senses that she is "different" from others. Seems like a high functioning degree of autism, or Asperger's Syndrome, but the series doesn't go into details.
Twirltongue is the true mastermind behind the Bane, and a vicious, manipulative xenophobe. Taking advantage of the Bane at a young age, Twirltongue uses her persuasion abilities to convince the unstable Bane that he will one day lead the Gnawers, with her as his adviser. Once setting the Bane up as king of an army of Gnawers, Twirltongue continues her manipulation and advising of the Bane, convincing him that every species that isn't a rat deserves to die, and to this end, the Bane's forces proceed to round up thousands of adult and children mice alike, then either force them to jump to their deaths by the hundreds, or shove them into horrific chambers and gas them by the thousands with volcanic ash. Twirltongue then has the Bane lead a full-scale war on Regalia, hoping to slaughter every man, woman, and child inside, then reign supreme over all other species of Underland.
Snare, despite his limited appearances, sticks out as the nastiest Gnawer in the story alongside Twirltongue. Once a general in the wicked King Gorger's army, thus facilitating the numerous atrocities committed by Gorger, Snare later gave birth to a litter of mice with his wife, Goldshard. Realizing that one of his newborn mice pups, Pearlpelt, is the legendary "Bane", a being prophesied to one day take over all of Underland, Snare promptly murders the rest of his litter to ensure Pearlpelt doesn't have to share his mother's milk with them. Snare takes to beating Pearlpelt and driving his hateful rhetoric into the baby, fully hoping to one day use his son to wage war on Regalia and ride the coattails to victory, and, when Goldshard finally has enough and tries to escape with Pearlpelt, Snare attempts to murder her in a blind rage. Though dead before Pearlpelt became the monstrous Bane, Snare fully facilitated his son's transformation into a psychotic monster, and would only laugh at the destruction he went on to inflict.
Fan Preferred Pairing: Even though the canon pairing is Gregor/Luxa, many fans seem to prefer spoiler:Gregor/Howard for some reason.
Or Gregor/Henry. Or Bane/Ares. Or Ripred/everyone.
He's Just Hiding: Many fans refuse to believe that Twitchtip is dead, mostly because Gregor (and the audience) only found out about her "off screen" death through a bit of intercepted information. Some say she faked her death and is out running in the Underland somewhere while others say the rats were just lying to each other.
What Do You Mean, It's for Kids?: Yeah, it's still a book series aimed at older children, even though it contains a massive ammount of extreme violence such as decapitation. Wounds are even sometimes described in explicit detail. Maybe the fact that it's written media and not shown as actual pictures keeps the Moral Guardians away.