Well, in the beginning, Harry may be resistant to Regulus' attempts to raise him. He has his own ideas and values from his short time at the Dursleys. But Regulus is patient. He starts by sharing small things about magic with Harry. As Harry realizes that Regulus truly cares about him and wants to protect him from the dark forces that once consumed Regulus himself, Harry starts to open up. Their relationship then blossoms as they go on adventures together, explore the secrets of Hogwarts, and as Regulus imparts his wisdom on how to be a good wizard and a good person. It becomes a relationship based on mutual understanding, respect, and love.
In the fanfiction, their relationship could develop in stages. Initially, Harry is confused by this new person in his life. However, Regulus shows his kindness by teaching Harry about his family history and the wizarding world. As Harry learns more about Regulus' past and his redemption arc, he begins to respect him. And as they face challenges together, like threats from the remaining Death Eaters or dealing with the Ministry of Magic's interference, their relationship deepens into a loving and trusting one.
At first, it might be a bit of a rocky start. Harry, having grown up without a proper family, could be wary of Regulus. But as time passes, he starts to see Regulus as a father figure. Regulus, on the other hand, is determined to make up for his past mistakes by raising Harry right. So he showers Harry with care and magical knowledge, and they gradually form a strong bond.
A possible plotline is the impact on Harry's personality. Being raised by Regulus, who was once a Death Eater but had a change of heart, might make Harry more complex. He could have a better understanding of the gray areas in the wizarding world. For example, he might not be as quick to judge Slytherins. And in terms of the overarching story, there could be a new take on the prophecy. Maybe with Regulus' influence, Harry interprets the prophecy in a different way that changes the whole course of the battle against Voldemort.
Obviously, Harry Potter and Regulus Black are the main ones. Harry as the boy who lived, and Regulus as his guardian, would be central to the story. Their relationship would be explored deeply.
A third plotline could be how Harry's Hogwarts experience is different. He might have different knowledge and skills compared to the original story. For instance, he could be more proficient in certain areas of magic that Regulus specialized in, like ancient runes or potion - making. This would give him an edge in his classes and adventures at Hogwarts.
The relationship develops gradually. Cloud, being from a different world, has to adapt to the wizarding world to raise Harry. Initially, he may be a bit distant, but as he witnesses Harry's struggles against the forces of the Dark Arts, he becomes more involved. He starts to share his own experiences of fighting against powerful enemies in his world with Harry. This sharing creates a deeper connection between them. Harry begins to look up to Cloud not just as a protector but also as a source of wisdom. And Cloud starts to care deeply for Harry, seeing in him a chance to do something good in this new and strange world he has found himself in.
At first, it might be a bit rocky. Harry, coming from the wizarding world, would be confused by Kakashi's ninja ways. But as time passes, he would start to respect and look up to Kakashi as a mentor. Kakashi, on the other hand, would see Harry's potential and determination, and grow to care for him like a student or even a son.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). Just type in 'Harry Potter and Regulus' in the search bar, and it will show you a bunch of relevant fanfictions. There are also some dedicated Harry Potter fanfiction forums where people might recommend good 'Harry Potter and Regulus' stories.
Fanfiction about these three can take many forms. For example, there could be time - travel fics where Harry goes back in time and meets Regulus and Sirius when they were younger. This could lead to all sorts of interesting plot twists, like Harry trying to prevent Regulus from joining the Death Eaters in the first place. Or it could be a fic where Sirius survives and he and Harry explore the secrets of the Black family together, including things that Regulus left behind. There are also fics that focus on the emotional bonds between the characters, showing how the different personalities of Regulus, Sirius, and Harry interact and influence each other.
One common theme is redemption. Regulus was once a Death Eater, and in many fanfictions, he tries to atone for his past sins through his connection with Harry. For example, in stories where he helps Harry uncover secrets about Voldemort.
It could become very close and trusting. They'd know each other inside out and have a deep bond from childhood.
The story would also shift in terms of Harry's place at Hogwarts. His sorting into Gryffindor might be more complex. Since he's related to the Black family, which has a history of being in Slytherin, there could be an internal struggle within Harry about his house loyalty. And his friends, like Ron and Hermione, might react differently to this new revelation about his parentage as they've always known him as a Potter.