Suggested by Sonic Rukia and Asta are both fighters who grew massively stronger as their series went on. Rukia certainly was captain level by the end of her adventure and Asta achieved that status in his verse as well. Ultimately I would give Asta the edge because he can keep up with Rukia’s speed and his anti magic should have a good effect against her ice. I would trust his ability a lot in that matchup and should have the edge in close quarters. Asta wins.