Since they are twins dennis and dee grew up at the exact same age right and they are both from a broken home and are unique people brought up around strange suburban horrors and were probaably eachothers best friends thier whole life. Even in higj school when they branch off they still spend all thier free time together with mac and charlie. For all the shit dennis gives dee he really loves hwr and wpuld kill for her because he sees her as the literal female veraion of him thier only main difference is dee somehow has hope and dreams whereas dennis develops a superiority complex and fetish

I think the reason dennis gives dee so much shit is because he does see her as a female cersion of himself and while being afraid of all intimacy he knows that dee can do so much better than the lifes they lead but he doesnt ever wants her to know that because while she is with him hes never truly alone

I have a theory that shelly kelly the perceeved father of charlie kelly died very shortly after they saw dee govered in bog muck, my theory is that shelly kelly took his own life because he was deathly afraid of what might be waiting in his future. As we know shelly kelly believes in the paranormal, other worldly things and he is a deeply old irish man and ireland have thick roots in wicca and mythology so i think that in his younger years probably after bonnie left him to go back to america he was down on his luck and decided to try doing thinga like wishes and deeds in fovor of myths and superstitians and stuff then one day his life started turning around so over the years he got more and more into it and things that he couldnt explain started happening to him and around him therefor when he saw dee not know who she is or where she came from thought that the time for some type of payment was coming for him