As of Skin Game, we know that Butters has accepted wielding Fidelacchius full-time. Murphy has picked it up off and on in the past, with varying results. Based on previous theory, to bear the sword on a regular basis, there is a requirement to have royal ancestry.
Did the books say from which royal Butters is descended? I can't remember. If not, what are the popular theories? He's Jewish, so it could go back to the kings of Israel and Judah, possibly even David.
What about Murphy? She never became a full Knight of the Cross, but she wielded it several times, and at one point, the sword was described as giving her a "job offer". Any theories on from whom she might be descended? Did they say in the books?
Other possible candidates for holding swords (based solely on lineage, not on temperament or character):
Thomas Raith. Descended from Lord Raith, ruler of the White Court for centuries. Royalty of a sort. Also, him taking up the Sword of Love would be interesting.
Marcone. His lineage remains completely unknown, but his position as an underworld lord and freeholding lord might count, making him the first of his line.
Daniel Carpenter. Same lineage as his father, duh. Seems a little on the nose, and I would rather see a sword go elsewhere, but it's a possibility.
Thoughts?