Their relationship would likely become very close. Harry, who has always longed for a family, would finally have someone who truly cares about him like family. Sirius, on the other hand, would get to be the father - figure Harry needs. They would probably have a lot of heart - to - heart talks, sharing their joys and sorrows.
At first, there might be some bumps in the road. Sirius has been through a lot and is a bit of a rebel. Harry is used to the rules at Hogwarts. But as they spend more time together, they would start to understand each other better. Sirius would teach Harry about the less - conventional aspects of magic, and Harry would show Sirius that there is still good in the world. They would grow to depend on each other, with Sirius protecting Harry and Harry giving Sirius a reason to be more responsible.
Their relationship could develop into a true partnership. Harry would see Sirius not just as an adult to follow, but as an equal in the fight against Voldemort. They would plan strategies together, with Harry contributing his knowledge from Hogwarts and Sirius his knowledge of the wizarding underworld. And in their downtime, they would bond over simple things like cooking together (even if Sirius is not the best cook) or taking care of the house at Grimmauld Place. This shared life would strengthen their bond to the point where they are more like brothers than anything else.
It could develop in a complex way. The spanking incident might initially create tension between them. Harry could be angry or resentful towards Sirius at first.
At first, it would be really rocky. Harry would be suspicious of Snape and Snape would be his usual grumpy self. But over time, as they get to know each other better, they might start to respect each other. Harry could learn about Snape's past and his complex relationship with his mother, which might soften his view of Snape.
At first, it might be a bit rocky. Harry is used to a more structured upbringing at the Dursleys', while Sirius is quite the free spirit. But as time passes, Sirius starts teaching Harry about his family heritage. They go on adventures together, like exploring the Forbidden Forest. Harry begins to see Sirius as a true father figure, and Sirius sees Harry as the son he never had. They also have arguments, like when Sirius is overprotective, but they always work through it, which deepens their relationship.
Sirius' character development could involve him coming to terms with his own past traumas while rescuing Harry. He may have to face his fear of Azkaban and the dark magic that he has encountered in his life. As he rescues Harry time and time again, he becomes more confident in his own abilities and his place in the fight against Voldemort. He also starts to see Harry as more than just James' son, but as an important part of the wizarding world's future, which makes him more committed to protecting Harry at all costs.
At first, they might be cautious around each other. Harry, having known Sirius, would be curious about his daughter but also a bit reserved. As they start to work together on small tasks in Hogwarts, like helping out in Dumbledore's Army - like activities, they'd begin to trust each other more. Eventually, they could become very close friends, sharing secrets and relying on one another in dangerous situations.
Harry would likely become more confident. With Sirius as his guardian, he'd have someone who truly believed in him from the start. Sirius was brave and rebellious, and these traits could rub off on Harry. He might be more willing to take risks and stand up for what he believes in earlier in life. For example, he might not tolerate the bullying from Draco as much as he did in the original story.
In the fanfiction, their relationship could be a complex one. They might have to deal with the prejudices of the wizarding society. Some might oppose their marriage because of Harry's half - blood status and Orion's family's past. However, they could use this as an opportunity to fight against such prejudices. They could support each other through difficult times, and as they do so, their relationship would grow stronger. Harry could introduce Orion to his Muggle - born friends, and Orion could introduce Harry to the more traditional pure - blood society in a new light. This exchange would help them understand each other better and solidify their relationship.
In some fanfictions, Sirius' character develops a greater sense of responsibility. When he rescues Harry, he realizes how important it is to be there for him, not just in times of physical danger but also for his emotional growth. So he becomes more of a mentor figure.
His relationship with Sirius would be extremely strained. Since Walburga was Sirius' mother and they had a very difficult relationship, Sirius would likely see Harry as a product of his mother's influence. Harry, on the other hand, might be initially hostile towards Sirius due to being raised with Walburga's views.
At first, their relationship would be filled with confusion. Harry might be shocked and Sirius would be equally bewildered. But as they start to accept the situation, they could develop a deeper understanding of each other. Sirius could become more of a parental figure to Harry in a different way, and Harry could offer Sirius a new sense of purpose.