DEATH BATTLE: Riku VS Acheron

Kingdom Hearts vs Honkai: Star Rail

Connections:

Both are dark, depressed heroes from famous games who are the close allies of their story's main protagonist (Sora and Trailblazer respectively)

Both of them once lived peacefully on their home worlds (Destiny Islands and Izumo respectively) until forces of darkness invaded and destroyed their homes (Heartless and Yaoyorozu no kami respectively) and in the process both would become slaves to major villains (Riku was possessed by Ansem, Seeker of Darkness and Acheron became the Emanator of IX the Nihility and followed the path of nihility)

Both are skilled in swordsmanship welding swords from their respective worlds (Keyblades and 12 Blades of Izumo,respectively) and contain swords that allows them to control and manipulate darkness (Braveheart and Naught respectively)

Both of them will go down a dark path feeling sadness and sorrow due to the circumstances of the past, but would have a bond with the protagonists, both of them also share a relationship with a wise veteran protagonist (King Mickey and Welt Yang respectively), as well as with shy and naïve girl (Namine and Firefly respectively), and would go on to assist the heroes in fighting major threats to their verses