Wg definicji poniżej, "ficelle" to termin ukuty wła¶nie przez Jamesa.
Chyba zostawiłabym w oryginale + przypis/wyjasnienie, że z francuskiego itd.

Ficelle character.
From the French word for 'string,' a term used by Henry James to denote a character who exists simply to move the plot or drama from place to place. In Shakespeare's "Hamlet," Rosencranz and Guildenstern are ficelle characters. Vladimir Nabokov called them peri characters.
http://sfwa.org/writing/glossary.html

ficelle
(French)device; James uses this word to indicate a character who is invented largely for the convenience of the author (compare Alfred Hitchcock’s ‘MacGuffin’ in his films)
(literally: a piece of string, particularly one used in to operate a marionette; or, a small stick of bread; also used in colloquialisms, which may have influenced James: ‘ficelle du métier’ = tricks of the trade and ‘connaître ficelle du métier’ = to know the ropes)
http://www.henryjames.org.uk/notes/ficelle.htm

ficelle - puppet string; James's term for a confidante
http://www.stanford.edu/~csewell/culture/litterms.htm (w "minor Literary Terms")

...Maria Gostrey, an important mediating character in the book – or ficelle as James would have it...
www.psa.ac.uk/cps/1996/osull.pdf

a tu sam autor o swoich "ficelles":
...Each of these persons is but wheels to the coach; neither belongs to the body of that vehicle, or is for a moment accommodated with a seat inside. There the subject alone is ensconced, in the form of its "hero and heroine," and of the privileged high officials, (xix) say, who ride with the king and queen... Maria Gostrey and Miss Stackpole then are cases, each, of the light ***ficelle***, not of the true agent; they may run beside the coach "for all they are worth," they may cling to it till they are out of breath ... but neither, all the while, so much as gets her foot on the step, neither ceases for a moment to tread the dusty road...
http://www2.newpaltz.edu/~hathawar/portrait1.html