Well, in the fanfiction, the power levels of both sides are crucial. The ancient humans in Halo may have had access to some extremely advanced, yet different from the Mass Effect universe, technology. The Reapers, with their massive size and powerful weaponry, would be a formidable foe. However, the ancient humans might have the advantage of being more adaptable due to their long history in the Halo universe. Also, the fanfiction could play around with the idea of alliances. Maybe some other races from either universe would side with the ancient humans or the Reapers, which would further complicate the story.
Toge Inumaki is important too. His special cursed speech ability makes him stand out. Also, the principal of the school, Masamichi Yaga, is likely to be important as he is in a position of authority and has an influence on Gojo and the other students. There could be other characters like the members of different clans that also play roles in the complex web of the story.
Perhaps the fanfiction could have a plotline where an ancient human scientist discovers a way to disrupt the Reapers' indoctrination abilities. This knowledge then becomes the key to the ancient humans' defense. Or, a group of Reapers could be sent on a secret mission to the Halo universe to study the ancient humans' Forerunner - related technology, which leads to a full - scale war when they are discovered. There could also be a plotline where a single ancient human, with unique powers or knowledge, becomes the main hope for the entire ancient human race against the Reapers.
Ash is always a popular choice. His long - standing relationship with his Pokemon makes him a great character for this kind of fanfic. Also, Pikachu, as it's one of the most well - known Pokemon.