Another way is to focus on their relationships. Have them interact with different characters from Highschool DxD. If they form a friendship with Issei, they could learn about human values from him, which would contrast with their demon upbringing. Or if they have a rivalry with another demon, it could push them to become stronger. Also, let their relationships change over time, like starting as enemies and then becoming allies.
You can also develop the character through their goals and motivations. Maybe their main goal is to restore the honor of the Lucifuge family. They could pursue this by learning new powers, making alliances, and facing off against enemies who have slandered their family name. Along the way, they'll face moral dilemmas, like whether to use dark magic to achieve their goals or stick to more honorable means. These internal struggles will add depth to the character.
One idea could be that the Lucifuge OC has a hidden power that gets awakened during a major battle at the school. Maybe it's a power related to time manipulation which could turn the tide of the fight against a powerful enemy.
Start with giving the OC Sitri a distinct personality trait. For example, if they are extremely curious, it can drive their actions and decisions. They could be constantly exploring the world of Highschool DxD and getting into all sorts of situations because of their curiosity.
Well, the character could be developed by first exploring his Shiva - given powers. Maybe he has incredible strength or some unique divine abilities. He could start as a bit of an outcast in the highschool setting of DxD due to his otherworldly nature. Then, as the story progresses, he gradually makes friends and allies, learning more about the world of DxD and how to use his powers in this new context.
First, you need to define their personality clearly. Are they slutty because of a past trauma or is it just their natural inclination? This backstory will help shape their character. For example, if it's due to a past betrayal, it can add depth to their promiscuous behavior.
The human OC can be developed by giving them unique abilities. For instance, they could have a special power that Rias doesn't have or understand. This would make them stand out and be a threat to Rias. Another way is through their backstory. A tragic or interesting backstory can make the readers sympathize with the OC and understand their motives for bashing Rias.
Sora's character can be developed by giving him relationships. For example, he could form a deep friendship with Issei. Through their interactions, Sora could learn about loyalty and friendship in the context of the Highschool DxD world. This would also allow his own values to be challenged and refined.
For the OC, you can start by giving them a unique backstory. For example, if they had a tough childhood in the human world before being thrust into the supernatural world of Highschool DxD, it could make them more reserved yet determined. Akeno, on the other hand, is known for her alluring and mysterious nature. You could develop her personality further by showing her more vulnerable side when she's with the OC, like how she might open up about her own past traumas.
Naruto in a 'Banished Naruto Highschool DxD Fanfiction' can be developed by exploring his reaction to the new culture. The Highschool DxD world has a unique blend of modern and supernatural elements. Naruto, who is used to a more traditional ninja culture, could be both fascinated and confused. He could start to question some of his own beliefs as he sees different ways of life. His relationships with the characters in DxD can also shape his character. For instance, if he falls in love with a character, it could make him more vulnerable and also more determined to protect. His power development could also be linked to his emotional growth, as he discovers new ways to channel his chakra in this strange new world.
For the OC, you can start by giving them a unique backstory. For example, if they had a difficult past that made them more empathetic towards Koneko's own struggles. Koneko's personality is already somewhat reserved and strong - willed. The OC can gradually draw her out of her shell by being patient and kind. Maybe the OC has a sense of humor that Koneko slowly starts to appreciate, and this helps in developing her personality to be more open and less stoic.
First, give your OC a unique backstory. For example, if your OC is a devil, they could have a tragic past that led them to become a devil. Second, think about their powers. Make them different from the existing characters in Highschool DxD. Maybe they have a special ability related to an element like ice or fire. Third, develop their personality. Are they shy, bold, or mischievous? This will make your OC stand out in the fanfiction.