Big things are happening on TV Tropes! New admins, new designs, fewer ads, mobile versions, beta testing opportunities, thematic discovery engine, fun trope tools and toys, and much more - Learn how to help here and discuss here.
Darren's friendship with Kurda: In the end of book 6, just after Kurda is executed and then again when Darren becomes a Vampire Prince, and he sees shades of his dead friends, he shows that he still thinks of Kurda as a friend after all that's happened.
Then later, in the end of book 10, the whole explanation regarding why Kurda was made into Harkat: in order to protect and support Darren, even by masterminding his own downfall.
And finally, in the manga adaptation of book 12, when Darren is in the Lake of Souls, and he feels himself being pulled out but he doesn't want to go, and he resists... but then Kurda pushes him out. A last act of motivation from a lifelong friend.