Personally, I agree with Blinkin. On top of that, I don't think any outsider would pledge eternal servitude to a mortal, regardless of who possesses their True Name or the result of the Charisma check (Tome of Magic has the information about true name use of an outsider in binding, if I remember correctly).<br/><br/>I'd say bind the succubus, give her full succubus powers and NPC gold, but require needing to rebind her every X days and giving her the kind of tithe you need to make her cooperate with you. Treat her as an NPC played by someone, rathe than a cohort or the like (and thus, she doesn't gain levels, for example).<br/><br/>If she stays with us for long enough and has reason to actually join us properly, she can find some way to plane shift from the Abyss to Faerun and start adventuring with us as a proper PC, and at that point we can start keeping track of her LA and ECL, give her PC wealth, and make her a proper PC in all the relevant ways. But... for system design reasons, I dislike the very concept of a cohort who's basically an extension of a PC, and thus effectively almost doubles the power that the individual PC brings to the table.<br/><br/>EDIT: While the Savage Species rules are a decent place to start, yes, we should use the 3.5 version of the race and not the 3.0 one. I believe there's an update for the monster progression somewhere... But that said, if we're keeping them as an NPC (like I'm suggesting), then there's no need to use the character progression rules unless/until they become a full PC.