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His mother could send the howler. In the wizarding world, mothers can be quite fierce when it comes to family reputation and she might be unhappy with Sirius's actions or friends.
I'm not sure of the specific Pfister Hotel ghost story. But often, in old hotels like the Pfister, ghost stories might involve past guests who passed away in the hotel and are said to still haunt the place. Maybe there's a story of a jilted lover who died of a broken heart and now her spirit roams the halls.
Perhaps another student, like Hermione, could send the Howler in a very creative twist in the fanfiction. Maybe she has found out that Draco is planning something malicious against the Gryffindors and uses the Howler as a way to threaten him into stopping.
His mother is a likely sender. She could be worried about Remus and send a Howler to remind him to take his Wolfsbane Potion regularly or to be more cautious around others. Mothers are often protective, and in the wizarding world, a Howler is a rather forceful way to get your point across.
It could be Professor Snape. He often had a contentious relationship with Harry, and if Harry did something that Snape thought was particularly wrong or against school rules in a way that affected Snape's interests, he might send a Howler.
It could be Severus Snape. He has a complex relationship with Dumbledore. Maybe Snape discovers some of Dumbledore's plans that he doesn't agree with regarding Harry Potter. He then sends a Howler in a fit of anger to express his disapproval.
Hermione could be the sender. If Snape has made some unfair judgment in class against Gryffindor students, Hermione, with her strong sense of justice, might send a howler to him to voice her protest.
Since I'm not familiar with the exact Pfister Hotel ghost story, in general, main elements could include strange noises that can't be explained. For example, unexplained moaning or whispering sounds. Also, there might be reports of things being moved around by an unseen force. There could be a backstory to the ghost, like it was a victim of a crime in the hotel long ago and now it haunts the place seeking justice or revenge.
The mystery. People are always drawn to the unknown, and ghost stories play on that. The Pfister has a long history, and with history often come these spooky tales.
The Howler might contain Harry's anger and defiance. He could be yelling about all the harm Voldemort has caused, like the deaths of his parents and many others. It could also include threats of how Harry will stop him once and for all.