Not too fluent in the Lone Man story (except through a cursory wiki dive), but I think I can help. If you want a Devil-figure (the Disney Method of working with mythology), then the Lone Man could be the shadow cast by the First Creator. He is, in essence, the First Destroyer and he brought death into the world. Of course, in the myth itself when the Creator asks his shadow why it created death, the Lone Man would say, "You were careless, and brought too much life to the world. Life multiplies too fast, and if I did not create an end to it, Life would become miserable when it used up all the land."
Or the Lone Man could be the brother of the First Creator, but I'd still have him create stuff like death in the above example. I think the original myth (read
here) had something to do with death coming into the world via a frog, but I just gave that job to Lone Man. I kinda like the idea of making some of the Natives a little Zarathustran in that they see a Good and Evil force at work in the world, but that neither is perfect. The First Creator created many good things, but also the bad, and he may be a bit careless in his wanton creation. The Lone Man is implacable as death itself, but he tempers his Brother's frivolities with cold, hard rationalism.