Maybe it's just a fun and affectionate nickname the author came up with to make their interactions more interesting.
It could be that Sirius views Hermione as having some kitten - like qualities. For example, she could be seen as curious like a kitten, always exploring new knowledge in the wizarding world. And Sirius, being an older character, might use this term to express his fondness for her in a somewhat teasing but loving way.
In the context of the fanfiction, it might be related to a specific plot point or character arc. For instance, if the story involves Sirius taking Hermione under his wing in some way, the 'kitten' nickname could symbolize his protective role over her. He might see her as a bit naive or vulnerable in some situations, similar to how one might think of a small kitten, and this name is his way of showing his concern and care.
It could be a term of endearment in the fanfiction world. Sirius might use 'kitten' to show a special, perhaps affectionate or protective, connection to Hermione.
Well, in some fanfictions, Sirius might be depicted as hating Hermione due to misunderstandings. Maybe he misunderstands her intentions or actions. For example, if Hermione is shown as being too strict with the rules in a story, Sirius, who has a rebellious streak, could resent that.
Well, Sirius has that kind of rebel charm. He's got the long, dark hair and a certain wild look in his eyes. In fanfiction, Hermione might see his confidence and his mysterious past as elements that add to his handsomeness. He's also brave and stands up for what he believes in, which can be really attractive.
There could be a fascination with power dynamics. Sirius is an older and more experienced wizard in the Harry Potter world compared to Hermione. Fanfic writers may be drawn to the idea of how his knowledge and power could interact with Hermione's intelligence and determination, thus leading to the concept of him dominating in the fanfiction. Also, it might add an element of unexpectedness or novelty to the relationship between two such different characters.
Often, it begins with Hermione trying to help Sirius deal with his post - Azkaban trauma. She uses her knowledge and compassion to reach out to him. As he heals, he starts to see her in a new light. Their relationship then grows as they start to understand each other better. The lemon parts usually are used to explore their more passionate and private moments, which are often written in a very tender and intense way, adding a new dimension to their relationship.
In some versions, Hermione might use her advanced magic knowledge. She could have discovered a spell that counteracts the pull of the veil. Maybe she found a way to disrupt the dark magic associated with it and was able to pull Sirius back from the brink just in time.
It's popular because it combines two very beloved characters from the Harry Potter series. Sirius is this cool, maverick character and Hermione is the brilliant, book - smart witch. Their combination creates a lot of interesting dynamics, like Hermione learning from Sirius's street - smart ways and Sirius being impressed by Hermione's knowledge.
Perhaps Hermione is always interfering in Sirius's business in the fanfiction. Like she might be trying to control how he interacts with Harry or the other characters, and Sirius, being an independent spirit, just can't stand it anymore.
In the wizarding world, there are always threats lurking. Hermione, with her inquisitive nature, sometimes gets into situations where she might be in over her head. Sirius, having a wealth of magical knowledge and experience, is seen as a perfect character to be her protector. Fanfiction writers like to play with this idea, showing how he can use his skills and instincts to keep Hermione out of trouble and safe from harm. It also adds an interesting layer to the relationships within the Harry Potter universe.
Maybe it's a way for Draco to express his intense dislike or misunderstanding of Hermione. Could be a plot device to create conflict.