Caius represents Etro’s heart and Hope represents Bhunivelze’s heart, they’re complete opposite sides which extends to Lightning’s own character since she tries to stop Caius and protect Hope, enemy and ally, just as Bhunivelze and Etro respectively. It’s also interesting how Caius represents the ally and Hope represents the enemy when both characters are the opposite to who they represent.
It’s also worth nothing that in Almighty Bhunivelze, there’s the call-back of the two final boss themes of the previous two games: Caius’ Theme and Fighting Fate. In this part of the battle, Bhunivelze is losing control and struggling with Hope’s soul.
Caius’ Theme is an oath to protect the loved one (ex: Dea Mortis, iuravi carissima servaturum which means Goddess of Death, I have sworn I will protect the most beloved = Yeul) and the order of timelines. Fighting Fate is a prayer for Bhunivelze’s return.
While The Ark -Hope and Bhunivelze’s fusion theme- plays, these themes are meant to symbolize the struggle between Hope and God.