Hearthfort has a very detailed imagination. If he watches TV he some times belives he is actually interactig with the characters or see and hear them. He also has a bit of a run away mind, basically he immerses himself in a reality he has seen or heard of to such extent that he actually believes its really happening. This is why he has therapy and has to watch what he watches or reads. If he ever wathes horror movies he will be so paranoid to that he will believe its real.
He already believes that the evil replica of himself named fluffy is real, and some times tgis condition goes out of control. The family he lives aith knows this, thats why they have hearthfort live in the basement with only his toys to entertain him.
A imaginary mind is good, but hearthfort gets too carried away. For example. Lets say its holloween and hearyhfort watches a horror movie, then he goes off to trick or treat in a costume related to the iconic scifi horror. He would get carried away and believe that he really is a monster and could hurt some one in this little shenanigan.
Hearthfort lives in a basement with little belongings, they do this to keep his acts under controll, the family refuses to go with drugs, so depriving him and therapy they chose. Hearthfort has no problem with this since he doesn't care.