One common character paired with Vergil is Dante. Some fanfictions explore a romantic relationship between them, which often delves into the complex dynamic of their brotherly bond and how it could transform into something more. It's a very unique take on their relationship.
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.
Nero is also a popular character to be paired with the reborn as Vergil. Given Nero's connection to Vergil in the original story, in fanfiction, their interactions can range from mentorship - where Vergil helps Nero master his powers, to more adversarial relationships if the reborn Vergil has different motives.
In fanfiction, female Vergil often has different emotional expressions. While the original Vergil is stoic and reserved, female Vergil might be more in touch with her emotions. For example, she could show more vulnerability when dealing with the loss of her family. Also, her fighting style might be more fluid and graceful compared to the original Vergil's more brute - force - like approach.