Harry was a poltergeist.
Because of course he bloody was. Severus had his suspicions when Harry had first formed as a ghost, so much energy even for a fully grown adult would make Severus think poltergeist. Potter luck was truly just a dismal thing, it is probably better the last of the Potter's was now dead so their continued line of luck couldn't bring an end to the world.
Now because Lily's son couldn't be normal in any life form, he also wasn't a typical poltergeist either. Which left Severus wanting some better books on ghosts and ghost lore, but with the Ministry's aversion to anything even slightly resembling necromancy, he should probably just be grateful that he found as much as he did.
Poltergeists were typically, classically, and until now only thought of as spirits of indestructible chaos. Being the only non-corporal beings to be able to physically interact with the living world. Ghosts can not move things that aren't already non-corporal. Poltergeists can, but generally don't have the calm or reasoning skills of a ghost either. So where did that leave Harry bloody Potter on the scale? There did not seem to be a classification that he could find that was in-between ghost and poltergeist. Demons and Inferi were too solid and generally raised by necromancers, although Severus did check just to cover all his bases.
How did Severus come to the firm and solid conclusion that Harry was in fact a poltergeist?
Well, ignoring his power levels, it wasn't a firm theory until Albus scared the boy and nearly got them both killed in the process.
—
"Harry, you are always welcome to hide in my rooms, but you might like actually hanging out with others of your kind." Severus tried to suggest when he found Harry haunting his kitchen after he had returned from classes that day. The response from Harry was a very firm and vigorous shaking of his head in disagreement, which just made Severus sigh. Ghost babysitting, how was this his life?
"Okay stay here for now then, just try to go out and interact with others once a day, please? Be they corporeal or incorporeal I don't care, but I am not the greatest resource of socialization for you." Severus tried to point out, but it was too apparent that Harry had already spent too much time with him in the last month when Severus received a droll look that he knew he often wore himself when told something inane. Severus caved to the inevitable and sat down in his favorite chair in the living room to grade papers.
Over the past month, Harry had taken to spending more and more time with Severus, or at the least, in his rooms. Too many people or too loud noises tended to scare the boy into disappearing. Severus had taken to spending his free time learning sign language so he could teach Harry to communicate with him and maybe others as well. He may have failed in protecting Harry who was now doomed to a life as a ghost, but Severus wanted to make Harry's afterlife the best he could manage. Severus had a brief knowledge of sign language from various students that had passed through the halls that had required it, but he needed to know more for Harry.
Severus was so deep in thought he didn't hear his floo activate or Albus stepping into his private rooms, without permission. His attention was pulled by Albus standing in front of him in his blinding orange and blue dancing star robes.
"Albus, why are you here?" Severus drawled reluctantly, ignoring the oncoming headache from looking at Dumbledore's robes too long.
"I just came to check on you my boy, and I hear Mr. Potter has been hiding in here with you," Dumbledore stated with a slightly disapproving tone. "Don't you think he should be somewhere else Severus?"
"He goes out with Helena when she shows up, but I believe that so many children in the school are overwhelming. Perhaps once school has let out for the summer he will come out more often to interact with everyone."
"Now that isn't good enough. Harry! My boy, won't you join me on a walk to the great hall for dinner." Dumbledore called out to Harry in such a loud voice as if he were deaf and not mute. The tone startled the boy into moving farther into Severus' rooms to get away from Albus. The fact that he didn't just disappear is something Severus would call progress, but it just seemed to agitate Dumbledore.
"Now Harry it isn't appropriate to be hiding in Professor Snape's rooms. Just come along with me and I am sure we can find you some friends!" Albus said following after the boy at a quick pace. Severus had a sinking feeling this was going to end badly.
"Albus I don't think-" Before Severus could finish, Dumbledore had caught up with Harry and was towering over the trembling ghost. Harry in a panic seemed to summon up all of his considerable power to get away from Albus and set off was looked to be a small magical bomb in the process, or at least that was what he was told as everything went black after that. A week later Severus woke up in the hospital wing with a raging headache and a very sad and apologetic little boy. It took two weeks to fix his rooms from the damage inflicted upon it.
—
Ever since that event, Harry had refused to be anywhere near the headmaster and very rarely left his rooms now. Severus did have some success in getting Harry to learn sign language although he still often reverted to making faces at Severus instead. Severus had a new respect for parents now though. Raising a kid was exhausting and Harry wasn't even alive. He wasn't sure if that made raising Harry easier or harder. On one hand, Harry couldn't eat so he also couldn't complain about eating vegetables as children often did. On the other hand, Harry was never going to get older, he may mature and learn, but he was forever a child and limited in some ways to a child's simple emotions and less complex thinking.
The Dursley trial was fast approaching and he wasn't sure how to explain his week-long absence to Harry. He just knew the boy was going to be upset that he had to stay in London for the trial. He couldn't even say the Dursley's name without having Harry blow something up in his rooms. Helena and the Baron already agreed to keep watch over Harry while he was gone. His only comfort was Albus was going to be at the trial as well and not in the castle to hassle Harry anymore without Severus to protect him. Hopefully, Harry would forgive him when he returned. The ghostly little boy had wormed his way into Severus' cold heart and Severus knew he could say with confidence that he loved Harry truly. Losing his trust would hurt, but Severus had lived with hurt before. He could do it again if he must.