It makes Harry more complex. He has to deal with the darker side of his new identity. He might become more reserved as he tries to figure out what it means to be the heir of Slytherin.
In this fanfiction, Harry's character development could shift towards being more self - aware of his own motives. The Slytherin side emerging might make him question his previous actions and reactions. For example, he might realize that he has been too hasty in some battles, and the Slytherin - like caution could be a new addition to his character. Also, his relationships with other characters, especially Slytherins, would likely change. He may find common ground with them and form new friendships or rivalries based on his new traits.
Harry may become more cunning as he is in Slytherin. He might be more guarded in his actions and words.
His character becomes more calculating. In Slytherin, he has to adapt to the house's values of cunning. So, he might think twice before acting, unlike his more impulsive Gryffindor self. For example, in a duel, he would strategize more instead of relying solely on courage.
It makes him more cynical. Harry, after experiencing abuse, is less trusting when sorted into Slytherin. He uses the house's cunning to protect himself from further harm.
In many fanfics, it completely changes Harry's identity perception. Instead of just being the Boy Who Lived, he now has this complex connection to Slytherin, which was considered the 'evil' house in the original. This can lead to different relationships with characters like Draco Malfoy. They might become allies instead of rivals in some fanfics.
It changes his character development in that he becomes more distrustful. Since he has the Dark Mark, he might start to doubt the intentions of those around him. He might think that people will always suspect him, so he becomes more withdrawn. Also, his sense of self - identity is shaken. He's no longer just the Boy Who Lived, but now he's someone with a mark associated with the darkest of magic. This could lead to him exploring his own morality more deeply. He might question if he's doomed to be like Voldemort just because of this mark, and he has to fight against that internal struggle to define who he really is.
He might start by promoting fairness and equality within Slytherin. In the original series, Slytherin is often associated with pure - blood supremacy. Harry, as head, could break that stereotype. He could encourage students from all backgrounds to work together and support one another, changing the cut - throat competition within the house to a more collaborative environment.
Well, if Snape is Harry's godfather in the fanfiction, Harry might become more complex. He could inherit some of Snape's cynicism and guarded nature. Snape, being his godfather, would likely influence Harry's view of magic, perhaps making him more studious in potions and more attuned to the darker aspects of magic. This could also lead to a more conflicted Harry, as he has to balance Snape's teachings with what he learned from his friends at Hogwarts.
In this fanfic, Harry's character development could be triggered by interactions with Slytherin characters. Their way of thinking and acting rubs off on him. He gradually becomes more ambitious and less impulsive. For example, when facing challenges, he no longer charges in head - first but instead thinks about the long - term benefits and consequences, just like a Slytherin would. His appearance might also change subtly to reflect this internal shift, perhaps his eyes taking on a more calculating glint.
Harry's character often becomes more shrewd and politically aware. In Slytherin, he has to be. He learns to use his wits in different ways than in Gryffindor. For instance, he might become better at reading people's motives and using that to his advantage.