Serana from Skyrim. She's my favorite companion, and ties for favorite character with Karliah and Cicero.
What an awesome vampire.
She really made me wanna side with the vampires in Dawnguard. Not because they're my favorite monsters, but also because she's a prime example that you can be a vampire, but you can also be relatively good. Anyway, she wanted to do the right thing, even if it meant killing her dad. I respect that.
Also, she's an amazing companion to have around. Except for the part where she constantly complains about walking around in the sun. But it's better than Cicero constantly singing about wanting to kill people. XD But her voice is nice and soft and really easy on the ears. So her complaining isn't all that bad.
[Fun fact: She shares the same voice actor as Lust from Fullmetal Alchemist.]
Though in all honesty, I don't think anyone could ever top Karliah's voice. Her voice is absolutely amazing to listen to.