“The Shaman” is how Tucker refers to the benevolent spirit of a Native American medicine man, whom Tucker met during his first spirit walk (cf. [Link]) at Poverty Point. The Shaman is a servant of Pautiwa, a benevolent god of the sky.
Although the Shaman guides Tucker with advice, he gives him only partial information. For example, in Season 1, the Shaman repeatedly contacts Tucker through talking crows, but reveals only in Season 2 that these crows are actually night crows, servants of Macha that the Shaman control temporarily only.
His motives for withholding information remain so far unknown.