Adventure is also very common. The OC usually gets involved in Killua's dangerous and exciting world of being a Hunter. They might be on a mission to find a rare item or defeat a powerful enemy. And there's often a theme of trust - building. Killua has to learn to trust the OC and vice versa.
One common theme is self - discovery. The OC often discovers new things about themselves while being with Killua. Another theme is friendship evolving into something more. For example, they start as friends or allies and then gradually develop romantic feelings. Also, there's the theme of overcoming differences. Since the OC is not from the original story, they usually have different values or abilities than Killua, and they have to overcome these differences to be together.
One common theme is adventure. Since Killua is from the Hunter x Hunter universe which is full of dangerous quests and exploration, many 'killua zoldyck x oc fanfiction' stories involve the OC joining him on these adventures. Another theme is friendship evolving into love. The OC and Killua start as friends or allies and then their relationship deepens over time.
Adventure is also a popular theme. They go on quests together, facing powerful enemies and exploring new places. This allows for a lot of action and character development.
One common theme is friendship. The SI OC often forms a deep bond with Killua, and they go through various adventures together, relying on each other. Another theme could be self - discovery. As the OC is in a new world with Killua, they learn more about themselves.
One common theme is power - up. When Killua crosses over into another universe, he often gains new abilities or finds ways to enhance his existing ones. For example, in a crossover with a superhero universe, he might learn to control a new type of energy. Another theme is friendship. He forms new bonds with characters from the other universe.
Another theme could be the relationship between Killua and the people around him, like his friends or family. For example, how Gon reacts to Killua's pregnancy and how they support each other. It could also touch on themes of love and sacrifice as Killua navigates this new situation.
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.
Adventure is also a common theme. The characters from Killua's world and RWBY's world going on quests together. They might be on a mission to stop a powerful enemy that threatens both their universes. This usually involves a lot of action scenes, battles against unique and powerful foes. And of course, there's often some element of mystery, like trying to figure out the secrets of the other world or the true motives of certain characters in the crossover.
Another theme is friendship. Killua is used to a certain type of friendship from his own story, and when he meets RWBY characters, new bonds are formed. It's about how he adapts to their group dynamic and vice versa. For example, his relationship with Ruby, who is very different from his friends back home.
Friendship is a very common theme. Since Gon and Killua have such a strong bond in the original work, fanfictions often explore different aspects of their friendship, like loyalty and mutual understanding.