Fakeking, just an idea, but how about if, in the Fate/DxD fic, you have it where Caster has not yet summoned Assassin and the MC does so, getting either Serenity or perhaps Jack (that type of thickness should not be underestimated!)
Follow along with the video below to see how to install our site as a web app on your home screen.
Note: This feature may not be available in some browsers.