A permanent resident is just that ~ someone who arrives from elsewhere into a host body and stays. I have only my own personal knowledge, and am contributing because the site requested it, so no doubt others will have other experiences and more to say.
In my sweetie's case, she and BEAR describe their early knowledge of and experience with each other differently.
BEAR arrived when she was five or six. He describes being in his first life, in the only other time and place he remembers, fighting the clan who had killed his wife and children. Then he remembers being violently killed, and *blip* suddenly being in a tiny, cramped, female body with someone attacking him and the body. As a natural fighter, BEAR did what he does best: fought. That began to solve my sweetie's problem of in-family abuse. The next few years were pretty rough for him as he acclimated and learned a new place, people, time, language, social customs, mostly from behind-the-scenes ... while mourning the abrupt cutoff from everyone and everything he had known up until then.
The first that my sweetie consciously knew of BEAR was more than a decade later. In police academy, she popped back in to the body and found herself holding her much-larger, much-more-experienced instructor pinned in a potentially-fatal position against the wall of the instruction gymn, with everyone staring at her in silence. She knew that something essential inside her had changed and would never be the same again, although she didn't know what. It took her a number of years to remember bits and pieces of BEAR helping her as she grew up, and to be able to hear/see/feel him clearly and listen to and talk with him.
Neither of them has a concrete answer for why BEAR arrived, but both agree that he helped her situation considerably. They live together almost entirely amicably; the only exceptions are over people one is attracted to when the other is not, and over tomatoes, which he loves and she hates. Watching them order food in restaurants and eat can be pretty amusing ~ blending does not occur much if tomatoes are on the table ...