I've never seen her hair before except in the shivering isles and I like it.
Is it an option of hair choice when creating a new character or is it just for NPCs?

Rena Bruiant's hairstyle is called "Quick Knot" and (on consoles) it's not playable.

It is called Dark Elf Topknot in the construction set (but is set to show as Quick Knot when you actually play the game). On the PC version all the races except Argonians, Khajiit, and Orcs can select it.

It is selectable by all races if a player on PC uses the mod: Oblivion Unlocker 2.0 by frank collins.
That mod unlocks:
all hairstyles - remove race locks and gender locks.
all eye styles - remove race locks.
all classes - remove PC non-playable class locks.

Jenifur Charne