First, you need to understand the basic elements of both Highschool DxD and the Saiyan concept. Incorporate the Saiyan's power and battle - thirsty nature into the Highschool DxD world. Maybe your OC Saiyan could be a transfer student who surprises everyone with their superhuman strength. Then, think about the relationships they will form. Will they be friends or rivals with the existing characters? Also, consider adding some unique Saiyan - related plot twists, like a special transformation that is triggered by a certain event in the Highschool DxD universe.
Well, one key characteristic is their battle instinct. Saiyans are born fighters, so your Highschool DxD OC Saiyan should have an innate ability to sense battles and be eager to participate. Their appearance can also be distinct. They might have spiky hair like typical Saiyans, and their eyes could glow when they get angry or excited. In terms of personality, they could be a bit hot - headed at times due to their Saiyan nature, but also have a soft side, especially when it comes to their friends. And their Saiyan heritage should play a role in their power - up. For example, they might gain more power when they are in a life - or - death situation, just like in the Saiyan lore.
First, you need to fully understand the settings of both Highschool DxD and the Super Saiyan concept. For the plot, you could start with the OC (Original Character) being a transfer student in the Highschool DxD world who has the power of a Super Saiyan. Then, you can create conflicts like how the OC deals with the different factions in the DxD world while hiding or revealing their Super Saiyan powers. Another idea is to have the OC's Super Saiyan powers attract the attention of powerful beings in the DxD universe, leading to various challenges and alliances.
The power level of the Super Saiyan should be a key consideration. Decide how strong your OC is as a Super Saiyan, whether they can reach different forms like Super Saiyan 2 or 3. Their appearance should also reflect their Super Saiyan nature, such as having golden hair and a powerful aura. In terms of personality, they might inherit the typical Saiyan traits like being proud and having a strong will to fight, but also be influenced by the Highschool DxD environment, so they could be more social or strategic in dealing with others.
An OC (Original Character) in a Highschool DxD fanfiction having the Rinnegan could be an extremely powerful and unique addition. The Rinnegan, from the Naruto universe, brings a whole set of abilities like the Six Paths powers. In the context of Highschool DxD, this OC could use these powers in various ways. For example, the ability to see chakra could be translated into seeing the magical energy in the DxD world. They could use the gravitational powers of the Rinnegan to manipulate the environment during battles against devils, angels, or fallen angels. It would make for a very interesting and overpowered character in the Highschool DxD fanfiction setting.
There are several out there. Some focus on how Naruto characters would adapt to the Highschool DxD world. For example, Naruto might use his shadow clones in the battles at the high school in the fanfiction. Another one to check out could be 'Saiyan Power in Kuoh Academy' which has some cool fight scenes and character interactions.
The uniqueness also lies in the potential for new story arcs. Since it's a crossover, the writers can take the story in any direction they want. They can create new villains that are a combination of threats from all three universes. For example, a villain who can use both Orochimaru's jutsu and the devious plans of the Highschool DxD antagonists, while also having Saiyan - like strength. This allows for a lot of creativity and makes the fanfiction genre really exciting.
The OC, as the Son of Sparda, could fit in by being a new and powerful entity. He might be initially seen as an outsider, but his powers would quickly draw attention. He could become involved in the power struggles between the different factions in Highschool DxD. For example, the devils might try to recruit him, while the angels might be wary of his presence. His unique abilities would also make him a valuable ally or a fearsome opponent. And in the high school setting, he could be a bit of an enigma to the other students, both human and supernatural.
One great fanfiction could be where the OC has a unique power that Akeno is initially intrigued by. For example, the OC might have the ability to control light in a different way than the devils in the Highschool DxD universe. This could lead to a series of adventures where Akeno and the OC team up to face various challenges. They could start off as just acquaintances but gradually develop a deeper bond as they fight side by side.
One way to incorporate different Saiyan forms is to base the transformation on the power - up systems in Highschool DxD. In DxD, characters often gain power through training, contracts, or special items. You could make it so that a character has to collect a certain number of dragon balls (in a fanfiction - inspired crossover way) to unlock a new Saiyan form. Or they might need to make a pact with a powerful being in the DxD world to be able to access Super Saiyan 3. This way, you blend the two universes' power - up mechanics in an interesting and unique way.