Ellis and the siblings are protecting Fiasse not because the plot demands it, but because she matters to them, and that weight carries through every fight. The action never overshadows why it exists. Four episodes is too short for what this story is trying to do, and the show knows it, so instead of rushing, it commits to the moments that count: the sword work, the singing, the childhood bonds that are the actual point. You watch this for the people, not the mystery around them.
Fiasse is a singer famous around the world and the headmistress of a special singing school that holds a charity world tour every year. After her mother's death, Fiasse was left an inheritance, and now some shady people are after both her and the money. Her childhood friends step in to guard her: Ellis, along with Kyoya and Miyuki, two siblings who are masters of the sword. With the next world tour approaching, Fiasse intends to go ahead with the show, and her protectors must keep her safe no matter the cost. The four OVA episodes blend adventure with romance.