I'm surprised people don't make a deal out of this: Darra was probably originally written as a player / tease / bad boy. We see this most obviously in the first two episodes of the first season.
1. The flirty look he gives Kyra, a complete stranger, as they pass each other at the juice place. (ep 01)
2. Immediately after, when Imogen hassles him about getting home late last night, he says he was at a wild goblin party. (ep 01)
3. When Kyra joins them for class, as they're flirting, Imogen pointedly interrupts by asking Darra about his date with the tree nymph. (ep 02)
4. When Imogen says she isn't trying to protect Darra from Kyra; rather, she wants to protect Kyra from Darra. (ep 10)
5. Also, there are a few places throughout the series when Darra teases Imogen and the girls, not in a flirty way, but in a mischievous way that we don't see consistently from him (e.g. "Ooh, Imogen misses the teacher").
It seems that this "teasing" side of Darra was written out, forgotten about, or just ignored in later episodes. Not sure why. Maybe because the show writers didn't want to focus too much on Darra + Kyra; the show is not actually a "teen drama," after all. Or maybe because Julian Cullen was a new actor and this type of character didn't look natural on him? I think Darra's personality probably became quieter to suit the actor. Anyone have other thoughts?