So far every Walker we've encountered has had a corporeal form. He Who Walks Behind, He Who Walks Before, and so on: they're all corporeal. Except for Nemesis, aka the Adversary, aka He Who Walks Beside. Nemesis corrupts people and ultimately possesses them, unbeknownst to others. We've never seen Nemesis directly, and so far in the books we're led to believe Nemesis doesn't even have a physical body.
I think I've figured out a strong possibility for (a) Nemesis having a physical form, (b) an explanation for why Nemesis is limited to possessing people, (c) where Nemesis is located, and (d) why nobody knows where Nemesis is and so many people think Nemesis must be immaterial.
Nemesis has made two, count 'em, two attempts at mounting a jailbreak at Demonreach. In Cold Days Nemesis was willing to see an asteroid-collision-level event happen if it meant jacking open the gates. In Battle Ground Nemesis provokes an actual for-real shooting war between the Fomor and the Accords and mortals just to provide a suitable diversion for the second attempt at opening Demonreach.
Nemesis has a special hatred for Harry, which so far everyone is attributing to him being starborn and thus the natural foe of Outsiders. But Harry is also the Warden of Demonreach Prison, who repeatedly tells us he's been too busy to review all of his prisoners (the vast majority of which he inherited) and what they're in for.
Guys.
Nemesis is imprisoned in Demonreach, and is trying to break out.
Think about it. It fits. Why does Nemesis have such a personal hate for Harry? Because Harry's his jailer. Why is Nemesis, such a powerful Walker, so bizarrely limited in what he can do/arrange? Because he's possessing people from inside a magical supermax cell. Why have we never seen Nemesis corporeally? Because he's in a magical supermax cell and can't get out.
Harry's searching for the Ultimate Bad Guy of the series and he's literally sitting in Harry's prison, and the reason Harry can't find Nemesis is ... because Harry's prison foils all tracker and locator spells.
Man, Harry, you really need to read your briefing packet. You may have turned your brother into Nemesis's cellmate.
Am I certain of this? No, of course not. But I do think it explains an awful lot.
Thoughts?