One idea could be a Neko OC who has a unique power, like the ability to control shadows. She could use this power to protect the main characters in secret, and as the story progresses, she gradually reveals herself and her motives. Another plot could involve a Neko OC from a different dimension who gets lost in the Highschool DxD world and has to find a way back while getting involved in the local supernatural affairs. Maybe she forms an unexpected alliance with a group of devils or angels.
The Neko OC could be a time - traveler. She comes from a future where the world of Highschool DxD has been destroyed. Her mission is to go back in time and prevent the events that led to the destruction. Along the way, she has to deal with the different factions in the present time of Highschool DxD, like trying to convince the devils not to start a certain war that will have far - reaching consequences in the future.
First, give your Neko OC a distinct personality. For example, she could be really shy but have a hidden feisty side that only comes out when she's protecting her friends. Second, give her unique physical features. Maybe she has heterochromatic eyes, one blue and one green, which is rare among Nekos. Also, a special ability like super speed or the power to heal others quickly can make her stand out. And don't forget to give her a backstory that ties into the Highschool DxD universe, like she was an experiment by a rogue devil scientist.
A plot could involve the mute OC being a new transfer student in the school. Their muteness makes them an object of curiosity and some characters might initially misjudge them.
One idea could be an OC who is a transfer student with a unique power, like the ability to manipulate time slightly. This catches the attention of the main characters in the Highschool DxD world. The harem could start to form as different characters are intrigued by the OC's power and mysterious background.
One idea could be an OC who is a transfer student with a unique power. The harem could form as different characters in the Highschool DxD world are intrigued by this power and gradually fall for the OC. For example, the OC might have the ability to heal wounds instantly, which is very useful in the battles that often occur in the DxD world.
One idea could be an OC demigod who has the power of a forgotten god. They could use this power to help the main characters in unexpected ways, like having the ability to manipulate time on a small scale during battles. Another plot could involve the OC demigod trying to find their place in the world of Highschool DxD, being half - god and half - something else. They might struggle with acceptance from both the devil and angel factions.
One idea could be an OC who discovers an ancient and forgotten form of Excalibur. This could lead to a power struggle among the various factions in the Highschool DxD universe as they all vie for control of this newfound power. The OC would have to navigate through political intrigue and dangerous battles to keep the Excalibur safe.
The OC could be a new transfer student with a mysterious past. As the story progresses, it's revealed that they are on a quest to find a powerful artifact that is somehow connected to the characters in the harem. This could lead to adventures together, and the building of deeper relationships within the harem.
One idea could be an OC Devil who has a unique power related to time manipulation. For example, they can freeze time briefly in battles, giving them an edge. Another could be an OC Devil from a long - lost noble family trying to reclaim their family's honor and status within the Devil society. They would have to face many challenges and rivalries along the way.
One idea could be an OC with a unique sacred gear that has powers no one has seen before. For example, a sacred gear that can manipulate time on a small scale, allowing the OC to freeze time briefly during battles in Highschool Dxd's universe. This would add a whole new dynamic to the battles and interactions.
An interesting plot could be an OC gamer who is known for their role - playing skills in the human world. When they enter the Highschool DxD world, they start to role - play as a powerful being. This act fools many of the characters at first, but as time goes on, they have to actually become strong to keep up the facade and end up changing the power dynamics in the DxD world.