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.
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.
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.
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.
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.
Hats in science fiction can often be a symbol of a character's identity. For example, in some cyberpunk settings, a character might wear a high - tech hat that not only protects from the elements but also has built - in communication devices or other gadgets. It can also denote their social status or occupation. A military - style hat might suggest a character is part of a space military force, while a more elaborate, flamboyant hat could signify a wealthy or eccentric character.
The evolution of hats in science fiction is closely tied to the development of technology in these fictional worlds. In the beginning, hats were mainly for protection or to denote a character's rank. But later, they started to incorporate things like energy shields or holographic displays. For example, in some recent sci - fi movies, there are hats that can project virtual maps or act as personal cloaking devices.
It could be Dumbledore. He was always one to oppose Voldemort and a howler from him would be a very interesting plot point. He might use it to taunt Voldemort or to try and make him second - guess his actions.
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.