Another way is to make the OC a long - lost relative of Fubuki. Maybe they have been separated since childhood due to some mysterious circumstances. When they meet again in the One Punch Man universe, there are a lot of emotions and they have to figure out how to reconnect while also dealing with the challenges of being heroes. This can add a lot of depth to the story and the relationship between the OC and Fubuki.
The OC can be introduced as a new recruit in Fubuki's group. For example, they could be a young and inexperienced hero who is in awe of Fubuki's powers and leadership. They start by following Fubuki's orders and gradually prove themselves useful in battles.
Si could be introduced as a new trainee at the Hero Association. He might start off as an underdog, constantly underestimated by the other heroes. But as he faces various challenges and battles, he gradually reveals his hidden potential. For example, in a battle against a powerful monster, he uses his unique skills, like being able to read the monster's movements really well, to gain an advantage.
A possible plot is that the OC is a former enemy of Fubuki who has seen the error of their ways. They try to make amends by helping Fubuki in her quest to become stronger and more respected in the hero community. This could involve training sessions where the OC shares some unorthodox fighting techniques that complement Fubuki's psychic powers. The OC could also have connections to some powerful and mysterious figures in the One Punch Man universe, which could open up new storylines for Fubuki.
The fanfiction about Fubuki and Tatsumaki in One Punch Man is alluring because it gives a chance to expand on their characters. Fubuki, with her ambition to be a great hero, can be explored further. Maybe in a fanfic, she overcomes her insecurities related to her sister Tatsumaki's much stronger powers.
First, think about unique powers. For example, a power related to manipulating sound waves in a very distinct way, like creating illusions through sound in the One Punch Man universe. Then, consider the character's backstory. Maybe he was an ordinary guy who got caught in an experiment gone wrong, which gave him his powers. Also, his personality should be engaging, like being a bit of a loner but with a strong sense of justice.
Many One Punch Man fanfics with Fubuki and Tatsumaki focus on their interactions with other characters too. They could be paired with Saitama in some comical or action - packed situations. Like, Fubuki trying to recruit Saitama into her group while Tatsumaki is constantly skeptical of his abilities.
One Punch Man is a really popular anime. Fubuki, as a character in it, has a lot of potential for fanfics. In a fanfic about them, it could explore their relationship in new ways. Maybe it could show Fubuki trying to recruit Saitama into her group in a more detailed and different way than in the original story. It could also develop her character further by showing her reactions to Saitama's overpowering strength in unique scenarios.
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.
In the One Punch Man manga, Fubuki is presented as a capable hero but also has her insecurities and struggles. Her character development is quite interesting as she interacts with other heroes and faces various challenges.
Fubuki is depicted as a powerful but complex character. She has her own ambitions and struggles.
It could work well if the OC's powers and personality are balanced and integrated seamlessly. The DC universe is vast, so there's room for unique characters.