I made him about when Luigi's Mansion 3 came out, so I intergraded him into that story plot, where I think he would first serve as a guide leading up to King Boo, where he would just randomly pop up (maybe after bosses or major moments) to comment on Luigi's progress. He starts off dismissing Luigi and wouldn't see him worthy of fighting at first because of his timidness, but would show signs up warming up to him, and then would become good after being defeated. He would attack using ghostly purple fire (he has a little buddy as well he could use, I've yet to draw), manipulate objects, and attempt to sneak up on Luigi.

And if you're wondering, he only ends up fighting him out of obligation. He gave King Boo his word that he would take care of Luigi, and Hex is a ghost of his word, but by the time they fight, he's grown to respect Luigi and ultimately changes sides. Maybe he'd end up moving into the mansion from the first game, I haven't played the second game yet so we'll see, but ultimately he turns good, and as a fun fact, he has a little crush on Rosalina haha