I wouldn't permit someone else to do so. Leadership doesn't work by magic. The cohort needs a reason to be their cohort. If they don't have one then the PC can call it a cohort all they like, but it's not loyal to them.
If you mean the shadowdancer, that would be redundant and a waste of a feat. It already does what they want. If you're asking whether or not it would advance, I would say no, simply because shadows are OP and not cohort material.
First a shadow dancers pet and a cohort are really two swperate things. Shadows are... not really people monsters while they have I think a 6? Int They seem to be described as cunning they are basically there to suck the strength of the living.
In short they make lousy sidekicks. Shadow dancer solves all of this by... ignoring it and having it as a class feature.
As a cohort you need to explain why it isn't killing the party or is loyal to anyone. Also be prepared for a covert stealth char almost no one will detect.
I wouldn't permit someone else to do so. Leadership doesn't work by magic. The cohort needs a reason to be their cohort. If they don't have one then the PC can call it a cohort all they like, but it's not loyal to them.
If you mean the shadowdancer, that would be redundant and a waste of a feat. It already does what they want. If you're asking whether or not it would advance, I would say no, simply because shadows are OP and not cohort material.
Don't they kind of advance anyway? I mean, like a familiar, they uses your BAB and saves. So the only difference from an actual shadow is that it only gets HP equal to half of your own (note to self: Shadow dancer barbarian with tons of CON, toughness, and favored class bonuses to health) and you could theorically get it to take class levels.
Which would be weird with class levels. It obviously can't do martial classes. And there would be problems as a caster, since it can't touch any material components. And that would be hardly the reason to take them in the first place, since you want their strength draining.
Flavorwise, I tend to think of the shadow as the shadowdancer's own shadow. It makes sense, since it shares their alignment, I have already mentioned how they share all your stuff like a familiar, and you risk being hurt when they are destroyed. I mean, it is like you literally have to grow a new shadow before you can summon a new one.
No, not at all. It already gets incorporeality, Strength damage, and progression. Letting it progress as a PC is ridiculous.
If he were in my game and he wanted to overwrite the shadow's stats with a weaker version of himself, that's fine. But it won't be incorporeal, undead, or cause stat damage. It'll just be a reflavored version of himself minus two levels and a lot of money. And he'll still have to make the Fort save to avoid getting a permanent negative level if it dies.