Imp sorcerer, pf.

[Duke of URL]

Imps don't have a listed level adjustment, which means that somehow, despite it not being particularly powerful, its level adjustment would be so high as to be unplayable. However, if you go by general 3.5 rules, it'd be at minimum ECL 3 due to hit die.

In standard 3.5, looking at the Imp's special abilities, especially the at-will SLAs, I'd be hard pressed to give it any less than an LA +3.

That'd leave it with 3 RHD and a +3 LA, for no sorcerer levels. I don't know what the Pathfinder rules for monster characters are, however. My own fixes for LA/RHD (which is a work in progress, and intertwined with a lot of other changes) would allow the character to cast as a 2nd level sorcerer at ECL 6.