The The Holy Grail of Eris, Vol. 3 (light novel) — Kujira Tokiwa

In her search for clues pertaining to Scarlett Castiel's framing for attempted murder ten years ago, Constance chances upon a mysterious invitation: a masked ball held by the anonymous Earl John Doe. If she wishes to attend, she'll need courage, cunning, and just a dash of wickedness reminiscent of Scarlett herself. Meanwhile, Randolph Ulster of the Royal Security Force is hot on Connie's trail, and if she doesn't wish for her crimes to be exposed, she must negotiate a solution with him fast. The perfect way to settle things is, of course…an engagement?!