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.
Fridge Brilliance - After a huge forest fire in the fifth book, Renn is wandering around the burnt Forest aimlessly. She starts running when she sees green in the distance, finally seeing trees. When she gets there she sinks to her knees and thinks about how the Forest is eternal and nothing can conquer it. Looking at the world's current environmental situation, it was brilliant.
Fridge Logic: Near the climax of the first book, Torak runs across a corpse and is very careful not to touch it, because it might anger the dead's spirit. Yet he shows precisely zero concern over lifting things from the man's body, i. e. looting the corpse. Er, what?
Foe Yay - Lots and lots in Outcast. All involving Seshru.
Tokoroths! They are very young children stolen from their families, enclosed in a cave, fed only rotting bat meat. Their teeth, fingernail, and toenails are filed to a point. They are kept like that until they have forgotten even the face of their mother. Then a demon is trapped inside of them. They then do crazy-evil stuff, at the bid of their creator. Renn is often said to have a "sharp-toothed grin", which is fine, until in Outcast Seshru reveals that she was preparing Renn to be a tokoroth, until Saeunn saved her as a baby... That just makes it darkly serious.
Seshru, Thiazzi, and Eostra. The demon bear. Eostra's dogs, especially when they attack Renn. Eostra's owl.
Wolf and Torak having to separate at the end of Wolf Brother.
Nef sacrificing herself to seal in the piece of the fire opal, with her last actions being to send her beloved bat away, so it doesn't die with her, and for Torak to tell his father that her debt to him is paid.
Torak forced to undergo the outcast ritual, where he's treated as if he just died. And then he's forced to run for his life.
Bale's sudden death in Oath Breaker.
Most of Ghost Hunter.
When Eostra's owl attacks the wolves.
When Torak says goodbye to Renn and kisses her.
Later in the book when Renn is thinking of continuing her life after Torak dies. He comes back to life, but when she realizes it, it's still a tear jerking part.
The last few, bittersweet pages.
What Do You Mean, It's Not for Kids? - Despite it mostly marketed as children's literature, there is a great deal of heavily described violence and gruesome images, as well as one or two important deaths in each book (also very gruesome and well-described). There is also some sensuality, especially in the final three books. And Thiazzi's physical and psychological torture of Renn in Oath Breaker
The Woobie: Dark, probably the most. His clan was disgusted by him because he was an albino, the only person who loved and protected him was his mother. When he was eight, his mother died. His father took him deep into the mountains and left him there. The next time he had human contact was when he was fifteen, and met Torak and, later, Renn.
Most of everyone else (Torak, Renn, and Fin-Kedinn) falls into Stoic Woobie.
Torak loses his father and is tossed into a destiny he was not prepared for, against foes he has no clue how to deal with. Having been raised away from the clans, he's not used to living with so many people, and feels like he doesn't belong with them. It doesn't help when he is later made outcast.
Oh, Renn. To start with, her mother is Seshru. Seshru only planned to use her as a tokoroth, at least before Saeunn intervened. Afterwards, Seshru didn't even bother to see if her daughter survived. She loses her father at an early age, and because of her talent for Magecraft, (which she is reluctant to use because she inherited it from her mother) people avoid her. Her first real friends were Torak and Wolf.