Imp sorcerer, pf.

[Rixx]

You use the creature's Challenge Rating (CR) as its total "monster levels" - and take the rest as Sorcerer levels.

Imps are CR 2, so you'd be Imp 2 / Sorcerer 4.

Halfway between party levels 8 and 9, take an extra level in Sorcerer, to compensate for your monster levels becoming less useful.

For the record, your ability score adjustments would be +6 Dex, +2 Int, +2 Wis, +4 Cha.

Level Adjustment doesn't exist in Pathfinder.