Well, Hisoka's origin story in Hunter x Hunter is quite interesting. He hails from Meteor City, a place that's more like a dumping ground for the unwanted. Growing up there, he was exposed to all sorts of things that normal people wouldn't experience. His love for fighting strong opponents probably stems from the fact that in Meteor City, strength was the only way to get what you want. Hisoka's nen abilities are also a product of his environment. He's constantly evolving and seeking new challenges, which is likely a trait he developed while growing up in a place where only the fittest survive.
There's a fanfic called 'Hisoka's Obsession with Machi'. It delves into Hisoka's perspective. He is constantly intrigued by Machi's skills and her cold exterior. The story has some interesting scenes where Hisoka tries to get closer to Machi, and Machi is both repelled and fascinated by him at the same time. It's a really engaging read.
One popular Hisoka fanfiction might be those that explore his backstory. Since in the original 'Hunter x Hunter' his past is quite mysterious, fanfictions often fill in the gaps. For example, there could be stories about his early days as a Hunter and how he developed his unique nen abilities.
In some fanfictions, Gon might initially be shocked. He has seen Hisoka as a somewhat eccentric but strong ally. When he discovers Hisoka's more sinister side, like his love for fighting to the death and his disregard for normal moral boundaries in battle, Gon could be conflicted. He might feel a sense of betrayal because he thought Hisoka was more on the side of 'good' in their adventures. However, Gon's own sense of justice and his unwavering belief in his friends could lead him to confront Hisoka about his true nature.
One great fanfic is 'The Unlikely Bond'. It delves into the complex relationship between Hisoka and Gon. The author beautifully captures Hisoka's twisted fascination with Gon and Gon's naivete in dealing with it. It has some really interesting plot twists that keep you hooked.
I really like 'Hisoka's Obsession'. It shows Hisoka's fixation on Gon in a more detailed and somewhat darker way. It adds layers to Hisoka's character while also developing Gon's reactions to Hisoka's strange behavior. The writing style is engaging and it's easy to get lost in this alternative take on their relationship.
A prevalent theme is also the idea of an unlikely alliance. The OC might form an alliance with Hisoka for various reasons, like a common enemy or a shared goal. And of course, there's the theme of self - discovery. As the OC interacts with Hisoka and other characters from 'Hunter x Hunter', they often learn more about themselves and their own capabilities. For example, in some fanfictions, the OC discovers hidden powers while facing challenges related to Hisoka.
The plot could also involve her having unique nen abilities different from Hisoka's. She might be on a journey to understand these powers and in the process, cross paths with the main characters from Hunter x Hunter. Perhaps she forms an unlikely alliance with Gon and Killua, who initially are wary of her because of her relation to Hisoka, but later come to respect her for her own qualities.
Fanfiction.net is also a great place. There are many 'Hunter x Hunter' fans who post their works there. Look for the section dedicated to the series and then narrow down your search to Hisoka - related stories. You can also check the reviews and ratings of the fanfictions to find the good ones.
One key element is his abnormal personality. He was born with a strange sense of what gives him pleasure, mainly fighting strong foes. Another is his lack of regard for normal social norms. He doesn't follow the usual rules, which allowed him to fully pursue his combat - related interests.