[Lawtan]

*Inserting one that left me in tears as a young kid.*
Gregor the Overlander.

They cut off all connections to the Underland in the end! They even killed Ares, my favorite character!
It was not "he's gone." It was "We had to cut off his rigor mortis claw holding your hand in order to save you." Oh, and he was your life-bonded giant bat, the guy who saved you again and again, the guy banished because his former master was a traitor, and he was the guy consoling the other bats through hard times...I felt like him...
For a 12-year-old, that's brutal. Add in that I read the whole series in 2 days while sick with the flu.