These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Alternative Character Interpretation: Senna. Even when she's the POV character for a book, she's so reserved and confusing that she lends herself to this trope. She herself barely seems to know who she is.
Anticlimax Boss: Anica, who turns out to be much weaker than the OMG POWERFUL WITCH that Senna had been hyping her up to be and gives up without even trying to fight after her plot with Merlin fails.
Badass Decay/Flanderization: David starts by seeming like a more BadassJake, even with/made more impressive by how he was a loser in the "real world", but his weaknesses eventually overtake him until mental weakness is more or less his defining feature. Unusual in that it arguably worked out for the better, letting the other three become stronger while he hit bottom. He does start to recover in the last two books.
Base Breaker: Everworld is obscure and has a small fanbase, but for what it is, Senna serves this role. She's either hated for being an evil, arrogant, manipulative bitch or loved for being an evil, arrogant, manipulative bitch. A third slice of the fanbase loves her for being an intelligent, competent Badass in a series where most mortals are merely the playthings of gods.
Hel gets this treatment, being portrayed as a malevolent sadist who lives to torture those who end up in her domain. She castrates her servants, and they are the lucky ones. Everyone else who comes to Hel gets buried up to their chin in rock, and has their head used as a cobblestone, until the skin is worn away and their skulls are crushed.
Ka Anor is the one being that the settings myriad Jerkass Gods fear. The god of the insectoid alien Hetwan, Ka Anor invaded Everworld with his army of mind slaved soldiers, intent on conquering the pocket universe and devouring all of its gods. During the siege of Olympus he offers Zeus the chance to escape with five chosen gods, if he will sacrifice the rest of his pantheon to Ka Anor's appetite; it is later revealed that Ka Anor had no intention of holding up his end of the bargain. His Hetwan legions violate every conceivable rule of warfare, even by ancient terms, using acid and fire as their main weapons. Ka Anor's single most horrific act, however, has to be his cannibalization of Olympian cupbearer Ganymede. Having captured the young god, Ka Anor takes the form of a swarm of insects, and strips him down to the bone, keeping him alive to the very last second, as the heroes watch in horror.
Interestingly enough, Senna is also an in-story example of a Draco in Leather Pants. April, Christopher and Jalil consider her to be a Devil in Plain Sight, but David spends the first five books trying to convince the others that she's good, and the next six trying to redeem her.
Ensemble Darkhorse: Senna. No matter your opinion of her, she's a multifaceted, compelling character whose presence alone is a good reason to read the series.
For all her selfishness, malicious tendencies, controlling personality, and generally being a bitch, Inside the Illusion makes it very hard not to sympathize with Senna.
David also qualifies, being an uptight Heroic Wannabe who's more adept at pissing the group off than actually leading them, while simultaneously being an insecure wreck, stemming mainly from his molestation as a child.
The other three too. They're all jerks on multiple occasions, but they're all going through so much that you can't help but sympathize. Jalil's arrogant, Christopher's a jackass, and April's self-righteous, but in their situations, would you do any better?
Launcher of a Thousand Ships: Not quite a thousand, but let's face it—April has some Ship Tease with all three of the boys (including, yes, David), and Galahad, and that's not counting the people she dates in the real world.
Magnificent Bastard/Smug Snake: Senna can be perceived as either or both. On one hand, she is extremely intelligent, Dangerously Genre Savvy, often successful at what she does, and inspires grudging respect from the core four. On the other hand, she is also quite arrogant, disdainful of others, extremely self-satisfied with her own intelligence, and can sometimes overestimate herself or underestimate her opponents.
Mind Game Ship: Perpetrated and exploited by Senna, especially towards David.
Moral Event Horizon: Ka Anor was a monster from the start, but his devouring of Ganymede nicely cements this. For Senna, there's Mystify the Magician, when she goes off the deep end and orchestrates the massacre of hundreds of people to feed her lust for power. As for Hel...pick a moment.
Powers a lot of the tension in the story. Ka Anor, in particular, is an insectophobe's anyone's worst nightmare. Ka Anor is Fear.
In-story example: The first thing Senna did with her witch powers was to disguise herself as Gollum, then sneak into April's bedroom while she was asleep and hiss out, "Wake up, my preciousssss. Is it awake? Gollum . . . Gollum . . . or is it dreaming, precioussss?" Poor April was only eight at the time, but holy crap did Senna ever get a kick out of that one.
Combined with an in-story example of Fan Disservice: Hel, who is deliciously sexy enough "in profile alone" to render the male characters incapable of thought. Until she reveals her other side, rotten, decaying flesh riddled with worms and falling off her bones. The worst part is that the guys are STILL helplessly attracted to her, even while being repulsed to the point of nausea. The detail in which she is described can only be called excruciating.
KAA loves this as always. Realm of the Reaper, might be the best example.
The Yeerk pool, Hel's domain, the Bosch painting...I'm sensing a pattern here.
Tear Jerker: For those who sympathize with Senna, Inside the Illusion will have some moments. For everyone else, there's Ganymede's death.
What Do You Mean, It's for Kids?: This series has a lot of violence, sexual references, curses, homophobia and alcoholism. So naturally it was displayed by stores and libraries in the children's section.
Word of Dante: Allegedly, the last three books were ghostwritten. Given general fan consensus to their quality, this would not be surprising if it was true. It especially makes sense because those three books overlapped with the beginning of Remnants, and according to Word of God, it was impossible to keep up Everworld and the occasional Animorphs book without a third series to complicate things.