He becomes more withdrawn. Harry was always a bit guarded, but now he's even more so. He doesn't open up as easily to new people or even his old friends sometimes. He's also more determined. The loss of Sirius makes him more resolute in his fight against Voldemort, as if he has a personal vendetta now more than ever.
After Sirius dies, Harry is in a really tough place. He feels like a part of his family has been ripped away. He may start to question his own actions that led to Sirius being in the Department of Mysteries in the first place. Harry will also feel a sense of loneliness, even with his friends around. But over time, he'll remember the lessons Sirius taught him, about being brave and standing up for what's right. This will give him the strength to move forward, and he'll use his memories of Sirius as a source of motivation in his fight against Voldemort.
In some fanfictions, Harry becomes more determined to fight against Voldemort. He channels his grief into training harder and becoming a stronger wizard. He often has flashbacks of Sirius which fuel his anger towards the Death Eaters and Voldemort, seeing them as the root cause of his loss.
Neville Longbottom could also play a more prominent role. Neville has his own growth arc in the series, and after Sirius's death, he could bond more with Harry over their shared experiences of loss. They could support each other in their fight against the Dark Arts. And perhaps some of the Death Eaters who were involved in Sirius's death could be main characters from the antagonist side, as Harry tries to track them down and bring them to justice.
One possible storyline could be Harry's struggle with his newfound loneliness and how it affects his magic. He might start to isolate himself more from his friends at Hogwarts, fearing that he'll lose them too. Another could be Harry's determination to avenge Sirius by seeking out the Death Eaters responsible for his death more aggressively.
In many fanfictions, Harry becomes more withdrawn after Sirius' death. He might lose his trust in the adults around him more easily, feeling that they couldn't protect Sirius, so he tries to be more self - reliant. He also could become more brooding and angry, his emotions more volatile as he grapples with the sudden and tragic loss.
Ron and Hermione become even more protective of Harry. They are constantly by his side, trying to cheer him up. Their relationship with Harry deepens as they share his pain. In some fanfictions, this brings Ron and Hermione closer together romantically as well, as they bond over their shared concern for Harry.
One common change is that he becomes moodier. The loss of Sirius is a huge blow, and he may swing between being extremely sad and angry. Another change could be that he is less likely to take risks without thinking. Sirius' death makes him more cautious as he realizes the consequences of recklessness.
A good 'After Death' Harry Potter fanfiction is 'When Harry Passed On'. In this story, Harry finds himself in a strange in - between world where he can observe the living but not interact directly. It's a very thought - provoking read. Another one is 'Death and the Boy Who Lived'. Here, Harry's death sets off a chain of events that forces the wizarding world to reevaluate their beliefs. And 'Harry Potter in the Aftermath of Death' shows a world where Harry's death has led to a split in the wizarding community, some believing he will return and others accepting his end.
The 'Harry Potter' books are set in a magical world full of its own stories, prophecies, and secrets. Sirius is a complex character with a lot on his plate. He has to deal with his family history, his time in Azkaban, and his loyalty to his friends. Fanfiction is a form of creative writing by fans outside the original story. Sirius doesn't have the time or the inclination within the context of the story to be reading fanfiction. His life is too filled with real magical drama and danger.