Well, there are fanfictions where Harry goes into a bit of a depressive state at first. He isolates himself from his friends, feeling that he's responsible for Sirius's death. But then, with the help of his friends like Hermione and Ron, he slowly starts to come out of it. They remind him of Sirius's sacrifice and how he wouldn't want Harry to give up. Harry then starts to focus on finding Horcruxes again, with a new sense of purpose.
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.
In many fanfictions, Harry becomes more withdrawn after Sirius' death.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.
Sirius could help Harry cope by sharing his own experiences of being an outcast in his family. He could tell Harry that just because his parents did this horrible thing, it doesn't mean he's unlovable. Sirius could also introduce Harry to his friends in the wizarding world, like Remus, who could also offer support and friendship.
Naruto might also isolate himself in certain fanfic scenarios. He feels that no one can truly understand the depth of his grief. He spends time alone, reflecting on all the things Jiraiya taught him and the memories they shared, like their time training on that toad -infested island.
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 some fanfictions, Harry's friends might take on more of a protective role. They see how broken Harry is after Sirius' death and they become more vigilant about his safety. They might also encourage him to open up more about his feelings. Harry, on the other hand, could initially resist their efforts but eventually come to appreciate their friendship more as he realizes they are just trying to help him through this tough time.
Maybe someone who witnessed Regulus' death tells Sirius the truth. This person could be a fellow Order member or a magical insider.
He might turn to his studies more intensively. Since he has no friends to distract him, he could pour all his energy into learning new spells and magic, becoming more powerful in the process.
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.
Sometimes Naruto copes by throwing himself into his training. He uses the pain of losing Jiraiya as fuel to become stronger.