I'm going to be playing through as a female Breton Assassin born under The Lady. Those are the things set in stone.

The suggestions I need are related to roleplaying. I'm interested in some back stories to get a feel for her tendencies. Anything within lore is welcomed. Will help me greatly as I've gone dry with creativity.

You could get a load of charm spells and control spells and be a siren of sorts. Having men follow you everywhere, fighting for you, etc...

There are some role-play suggestions in this 'character database ' link:

http://www.gamesas.com/index.php?/topic/939912-oblivion-character-database/

some class names are repetitive, which does not mean they play the same.

Pick a race for her to despise, perhaps members of said race killed her family or her lover.

Actually, try this idea.

Khajiit (can be replaced with anything) murdered her lover while she was out of town. When she arrived back to their house, all she saw was her lover's dead body. After the guards cleared everything up, fur was noticed all around the house, pointing to Khajiit assassins. This prompted her to take revenge on any Khajiit she meets, beginning her career as an Assassin.

I know it's woefully undescriptive, but it's just a basic outline to give you an idea.