One idea could be an OC who is a keyblade wielder from a long - lost world. They meet Ventus during his travels and join him on his journey to restore the balance in different worlds. This OC could have a unique keyblade design inspired by their home world's culture.
To write a believable fanfic, you need to develop your OC's personality. Are they brave or cautious? Optimistic or pessimistic? Their personality should influence their actions and decisions. When it comes to Ventus, show how your OC's presence affects him. Maybe your OC's positive attitude helps Ventus overcome his self - doubts. And don't forget about the dialogue. It should be in - character for both the OC and Ventus.
I really like the fanfic 'Terra and Ventus: A New Beginning'. It starts with an interesting take on their first meeting. The author does an excellent job of showing their personalities - Terra's determination and Ventus's innocence. It also has some really cool action scenes that are in line with the Kingdom Hearts style.
Perhaps the OC has a special power that is linked to Ventus' past in some way. For example, she can see glimpses of his forgotten memories. This leads to a journey where they explore his past together, while also growing closer and falling in love along the way. They might face challenges from Organization XIII who are interested in the OC's power.
First, make sure their personalities complement each other. Ventus is kind - hearted and brave. So, if the OC is more cautious and analytical, they can balance each other out. For example, Ventus might rush into a fight, and the OC can stop him and think of a better strategy.
One common theme is self - discovery. Since the OCs are new to the Kingdom Hearts world, they often have to figure out who they are and what their place is. Another theme is friendship, as they usually team up with the main characters from the game and form bonds. Adventure is also a big theme, as they travel through different Disney - inspired worlds.
The exploration of their individual characters. In fanfics, we can see different sides of Terra and Ventus. Ventus might be shown as more than just his innocent self, having hidden strengths. And Terra's more complex nature, with his struggle between light and darkness, can be further explored.
One good one could be 'The Bond Beyond Darkness'. It really delves into the complex relationship between Ventus and Vanitas. It shows how their connection is more than just their opposing natures.
In fanfiction, when Sora meets Ventus, they might immediately recognize a connection. Since they are both linked to the same heart, there could be an exchange of memories or feelings. Sora might feel a sense of familiarity and warmth towards Ventus, and Ventus could see Sora as a kindred spirit.