Friendship or brotherhood is also a big theme. Despite their differences, many fanfictions show how they support each other in their own ways, like Killua relying on Illumi's skills in certain dangerous situations and Illumi having a soft spot for Killua deep down.
Love and loyalty are themes that are often explored as well. It's not always romantic love, but a deep, unwavering loyalty that might exist between them. Some fanfictions show how, despite their differences, they would still be there for each other in difficult situations, like when facing a common enemy or dealing with the challenges within their family.
One interesting fanfiction could be about Killua and Illumi's relationship evolving. Maybe Killua is trying to break free from Illumi's over - protective and controlling ways. They could end up having a heart - to - heart conversation where Killua makes Illumi understand that he needs to grow on his own terms.
A common theme is the conflict between their different paths. Killua wants freedom and to make his own choices, while Illumi is bound by the family's rules and expectations. Fanfictions often explore how they deal with this divide.
One popular illumi and killua fanfiction is 'A Brother's Bond'. It delves deep into their complex relationship, showing how they interact beyond what's shown in the original work. It has some really interesting character development for both of them.
There's 'Illumi's Obsession'. In this fanfic, it delves deep into how Illumi's feelings for Killua are complex and often misunderstood. It shows how their interactions can be both intense and full of unspoken emotions. It's quite engaging as it plays with the psychological aspects of their relationship.