There are quite a few. Some fanfics focus on individual Slytherin students who form an unlikely bond with Harry. They might protect him from the dark forces that are after him, seeing him as more than just the 'Chosen One' but as a friend in need. These fanfics often explore the idea that not all Slytherins are bad and can be quite heroic when it comes to protecting someone they believe in.
In abuse - related fanfics, Slytherins being protective of Harry can be a way to explore different character dynamics. It breaks the mold of the typical Hogwarts houses' relationships. Slytherins might be written as having a hidden soft side that emerges when they see Harry being abused. It could also be that the author wants to create a more inclusive and diverse story where traditional enemies can become allies.
One reason could be that they see Harry as a powerful ally. They know his skills and abilities, and they want to be on his side.
There's also 'Slytherin's Shield'. Here, Harry protects the Slytherins during the war. He forms an alliance with some of the more noble - minded Slytherins and uses his influence to keep them safe from Voldemort's wrath as well as from the suspicion of the other houses. It's a great read for those who like to see character development and unexpected alliances.
One popular 'Harry Defends Slytherins' fanfic might be 'Slytherin's Champion' where Harry stands up for his Slytherin friends against baseless accusations. In this fanfic, he realizes that not all Slytherins are bad after getting to know some of them personally.
Another theme is the idea of hidden alliances. In many of these fanfics, the Slytherins form a secret group to protect Hermione. This adds an element of mystery and intrigue as they work behind the scenes to keep her safe from threats, both inside and outside Hogwarts.
Well, in fanfics, Harry might protect the Slytherins because he realizes that they are not all like the Death Eaters. He understands that many Slytherins are just kids who are sorted into a house with a bad reputation. For example, he could have befriended a Slytherin who was kind to him, and that made him see the whole house in a different light.
There's also 'Sirius's Oath'. It shows Sirius making an unbreakable vow, in a way, to always protect Harry. It delves into their relationship, with Sirius teaching Harry things he never learned at Hogwarts, all while shielding him from the dark forces that are after him. It's really interesting as it also shows how Sirius's protectiveness affects Harry's growth as a wizard.
Well, in some fanfics, Slytherins might be protective of Harry in an abuse scenario because they see the injustice. They could be written as having a sense of fairness that overrides the typical house rivalries. Maybe they've come to understand Harry's situation better than others and feel compelled to protect him.
The themes often involve loyalty. Slytherins in these fanfics show loyalty to Harry in a way that goes against the stereotype of their house. Some fanfics also have the theme of family. Maybe a Slytherin character sees Harry as a surrogate family member and will do anything to protect him. There are also themes related to justice, where Slytherins protect Harry because they know he's the key to a just future for the wizarding world.
There's also 'The Slytherin Guard for Hermione'. This fanfic shows how some Slytherins, who have grown tired of the house's bad reputation, choose Hermione as the symbol of their change. They protect her from dark wizards who target her for her Muggle - born status. It's an interesting take on how Slytherins can be allies.