To integrate well, the OC should have a clear backstory that ties into the lore of High School DxD. If the OC has a connection to an ancient artifact or a long - lost magic that is relevant to the world's history, it makes their presence more believable. Also, they should have goals and motivations that align with or conflict with the existing characters' in an interesting way.
The OC can have pre - existing relationships with some of the minor characters in the High School DxD world. For example, they could be an old friend of a side character who then gets introduced to the main group. This way, it feels more natural for them to be part of the story.
One way could be through the power system. The EMS (Eternal Mangekyou Sharingan) from Naruto could be a unique power that a character in the High School DxD world acquires. Maybe a transfer student from a hidden ninja village arrives at the high school and has to keep their EMS a secret while dealing with the devils and angels in the school. This could lead to some interesting battles and interactions as the powers of the EMS, like the ability to use powerful jutsus, contrast with the magic and abilities of the High School DxD characters.
The OC could have a special power where they can analyze opponents like in a fighting game. They can see weaknesses and strengths, just as a gamer would study an enemy's moveset. For instance, when facing a devil, they can quickly assess if the devil is more vulnerable to physical or magical attacks.
First, give the male OC a believable backstory related to his past life and how it ties into the BNHA world. For example, if he was a scientist in his past life, his knowledge could be used to develop new support items for heroes. Second, have him interact with the existing characters in a natural way. He could start as a transfer student at UA and make friends or rivals with the canon characters. Third, give him a unique quirk that both fits the BNHA universe and is related to his past - life experiences.
Asta could be brought into the High School DxD fanfic as a result of a multiverse - related event. In this case, different universes are colliding or being connected in some way. His presence would disrupt the power balance as his anti - magic is something that no one in the High School DxD world has really encountered before. He could end up training with the devils to learn more about their magic while also teaching them about the concept of anti - magic.
A well - written one often has a believable oc character. For example, their gaming skills should be integrated realistically into the High School DxD world. Their reactions to the strange events and characters in DxD should be in line with what a gamer might do.
To integrate an OC well, first, consider their personality. If the OC is a confident and brash character, they could contrast with Android 21's more complex and often reserved nature. Second, think about their goals. If the OC's goal is to find a cure for a certain disease that Android 21 might be related to in some way, it creates an instant connection and a plot thread. Third, make sure their appearance also fits the DBZ aesthetic, like having a unique fighting gi or a distinct hairstyle.
Give the OC a unique skill or trait that is relevant to the Sherlock universe. It could be something like an eidetic memory or a talent for deciphering ancient codes. This way, they can interact with Sherlock and the other characters in a meaningful way, either as an ally or an adversary.
Another way is to make the OC wizard an ally of the Huntsmen and Huntresses. They could assist in battles against the Grimm. For instance, the wizard could use their magic to heal the wounded or create illusions to confuse the Grimm. This would make them a valuable asset in the fight against evil in the RWBY world.
An OC can be integrated by having a relevant skill set. For example, if they are a mechanic, they could be the one who keeps the crew's cars in top - notch condition. This would make them an essential part of the team and give them a reason to be around during all the action.
A fanfiction could focus on Naruto's training regime influencing the characters in High School DxD. His intense physical and mental training could inspire the students at the school. They might start incorporating some of his training methods, like running up trees or on water, to improve their own strength. And Naruto could also learn from their magic-based training and combine it with his chakra control for new and powerful techniques.