People such as shamans act as mediums for spirits & willingly let them enter them & take on their personalities & some features (but only from the time of death and depending on how muchl the spirit is in control). In this state it is best for the spirit to have the same goals as the shaman or vice-versa to achieve 100% integration, that or the spirit has to be totally submissive to the shaman (meaning the shaman would only be using the spirit as a tool, basically using them only for their abilities like Tao Ren used to do).