The Promise Begins with Kuuga

"Whether it's protecting smiles, protecting humanity, or standing by any other conviction, as long as it's something we promised each other, I'll make it happen. Because we... made a promise, didn't we?"

This is the story of a red-clad swordsman traveling the world and holding fast to a promise he made.

(The protagonist is not KUUGA, but Saber. No harem, one female lead.)

Worldline: Kamen Rider Kuuga -> FGO -> Puella Magi Madoka Magica -> No Game No Life: Zero