I think vampire would be the best fit. Pass as normal, good for thieves and daggers. The gnome part would be a little weird, but just go with it. The only real problem would be having to adventure at night, but it's surmountable--witness all the tv shows and books with vampire characters--if the other players are willing to go along with it.
I would recommend finding a decent vampire prestige class on the internet, and making the character multiclass into it. That will mitigate most of the power concerns.
I would recommend finding a decent vampire prestige class on the internet, and making the character multiclass into it. That will mitigate most of the power concerns.