Wasn't there a way in Book of Exalted deeds to basically "reform" something evil and change it's alignment over time?
If you could shift it all of the way to neutral good...hmm.
Now, even if it had a change of life and thought better of its demonic ways, wouldn't it still be a Demon? Hmm...then maybe you could have a reformed Demon for a cohort.
Of course, it'd be much easier with a Devil, you only need to shift it's alignment one place from Lawful Evil to Lawful Neutral.
Interesting question...