Snape's personality would likely soften. He'd have to be more patient and understanding. Instead of just being the strict and often sarcastic Potions master, he'd become more nurturing. He'd still have his sharp edges, but around Harry, he'd show more kindness and care as he takes on the role of a guardian.
One possible plot could be that Snape, out of a sense of guilt or a newfound understanding of Harry's situation, decides to take him in. Snape would have to deal with Harry's grief over his parents and his own complex feelings towards the boy. He might start to teach Harry magic in a more personal and different way than at Hogwarts, leading to a unique bond between them.
In such a fanfic, Snape's parenting style might be strict yet protective. He would likely enforce high standards in Harry's magical studies, just as he does in the classroom. But at the same time, he would secretly keep Harry safe from any real threats. He might not show much affection openly, but his actions would speak volumes.
Snape's character changes in his relationship with others too. He becomes more involved with the Hogwarts community as he is now responsible for Harry. He might start to collaborate more with other teachers, like McGonagall, who he previously had a rather cold relationship with. His attitude towards the students also changes as he realizes that they are just kids trying to find their way, much like Harry. This is a big shift from his usual strict and often unforgiving self.
In this fanfiction, the story could start with Harry finding Snape in a very vulnerable state after the Battle of Hogwarts. Harry feels a deep sense of guilt for how Snape was treated throughout his life. So he decides to adopt him. At first, their relationship is awkward. Snape is still his old sarcastic self, but Harry is determined. As time passes, they discover they have a lot in common. They both lost their parents at a young age, and they both had to face great challenges. This newfound understanding leads to a transformation in their relationship. Harry helps Snape to come out of his shell of bitterness, and Snape teaches Harry more about the Dark Arts in a controlled and ethical way so that Harry can better protect the wizarding world in the future.
The relationship would start off rocky. Snape would be resistant to Harry's kindness as he has always seen Harry as James Potter's son. But Harry's persistent efforts to understand and help Snape would gradually break down Snape's walls.
In such a fanfiction, Harry might see the potential in young Severus that others overlooked. He could protect Severus from the bullies at Hogwarts. Harry, having experienced his own share of hardships, would understand Severus's situation to some extent and try to give him a better childhood, perhaps even guiding him to use his magic in a more positive way.
A really unique plot could be Snape adopting Harry during a time when Voldemort is still a threat. Snape has to balance protecting Harry from Voldemort's followers and also from the prejudice within the wizarding community. He might train Harry in Occlumency more intensively, and in the process, they develop a deeper understanding of each other. Harry starts to see Snape not as the mean professor but as a complex person with his own traumas and reasons for his actions.
Severus' character undergoes a major transformation. He begins to value relationships more, especially his newfound father - son relationship with Harry. He may start to interact more with the other students in a less intimidating way because he realizes that being harsh all the time isn't good for anyone, especially Harry. He also becomes more involved in the non - potions aspects of Harry's life at Hogwarts, like helping him with his other studies or dealing with bullying from other students. This shows a side of Severus that was previously hidden, a side that cares deeply for someone other than himself.
The two main characters are obviously Severus Snape and Harry Potter. Severus is the one who adopts Harry, changing from his usually cold and aloof self to a more caring and protective figure. Harry is the abused child who gets a new chance at a better life under Severus's care.