Unless the Demandred-short or notes give some really cool Fain details we're left with a character I'm half-convinced was post-CoS retconned by RJ into a red herring just to spite fan theories. Any, any, of the popular Fain theories would have been better than that waste.

Fain - an initially contemptible if somewhat tragic Gollum-esque character who never did anything important to the plot after aCoS and really hand't mattered since tSR. Rand's extra wound on the wound didn't really serve any important story purpose and one of the Ashaman traitors could have battled him in fashion that ended up with him locked up in a cell. Fain never even came close to influencing Rand's actions after that, or Moridin and the DO.

He's a lot like the other 'weird powers' baddie, SH, who shows up as a badass by punking around the Forsaken, then falls over dead for no reason. Oh, the DO outgrew him, which would make a lot more sense if the DO had flown in as some fifty-foot high Lovecraft-style abomination, as opposed to... nothing.

Fain should have replaced the DO afte Rand killed him, fixing the whole DO keeps making the same mistakes conundrum, or killed Moridin in a moment of body-switching confusion and near-victory-for-the-shadow, or even skipped the zombie hoard and Eldritch abominaiton phase to have him sneak into the camp while Moridin and Rand were dying and killed Rand's body.

So Fain was just a wasted character, RJ should have off'd him circa tSR because it was the last time he did anything significant except by screwing up, and that not much and strictly to the benefit of the Light.