This might belong in the quarantine section, since it covers the latest book. Not sure how up to date everyone in this part of the board is.
To be able to play a character like this, all I would require is an aspect and at least one point of remaining refresh, just like any other character.
When it comes to Grey, I think the "Rent" is simply due to his heritage. He is, after all, a scion of a naagloshi, and from what we know, they need to stay on tribal land to have their full power or maybe even to be able to exist. Now Grey might get out of the second one due to being part human, but he will have the same limitation on his powers as the naagloshi. He needs to be on tribal land or at least regularly visit tribal land to charge up his powers. Now from how he talked about it, it doesn't seem to be that there is much love for his naagloshi parent, which to me would mean that he would stay as far away from tribal land as possible. Yet he has full control of his powers.
To me, that says that he is cheating. He's got access to tribal land without having to fear his parent. Maybe he even carries tribal land with him in some way, so he can always act as if he is standing at the source of his power.
That probably involves a deal of sorts. Maybe with one of the queens. Another naagloshi. A god. Who knows? But in order to be allowed to continue tapping into that power, he needs to pay rent.
Now Vadderung is a bit odd. Now everyone assumes that he is Odin. Fair enough, all the signs are there. But for the purposes of this, I would propose another idea: He was Odin. He had the Odin mantle, but he gave it away. Or destroyed it. Or transformed it. Or bargained it away. In any case, he is basically a very powerful wizard now, maybe with some remnants of the Odin mantle still with him, some knowledge, for example, but not the full Odin mantle. Maybe the god mantles had made a deal that wouldn't allow them to inject themselves into certain conflicts. To get around this, Vadderung put down the mantle that would restrict him and only kept mantles that wouldn't hinder him in pursuing his goals. As a player character, you probably have to dial him down quite a bit. He'd be a full wizard at least, and then some.
Maybe, Vadderung even split up his Odin mantle among his children and their children and so forth, letting the mantle go into the bloodline, as it were. His comment about his children could hint at that.