Well, this is something that's entirely mine. I studied the personalities of each major character, then I studied the stereotypical qualities of each sign, and decided which signs best fit each character. Useful if you need to give a character a birthday. :) I did include for you the dates of each sign. Grantaire is the only character not given one because he does not seem to fit any of the signs. I will have to do a more in-depth study of the signs to give him one, since my source used was just a quick list of keywords for each sign.

BTW, the colors refer to the Element each sign corresponds with. Blue for Water, brown for Earth, yellow for Air, red for Fire.

Jean Valjean: Scorpio (October 24 -  November 22)
Javert: Taurus (April 20 - May 20)
Fantine: Pisces (February 20 - March 20)
Cosette: Cancer (June 21 - July 22)
M. & Mme Thénardier: Capricorn (December 22 - January 19)
Eponine: Capricorn (December 22 - January 19)
Gavroche: Gemini (May 21 - June 20)
Marius: Pisces (February 20 - March 20)
Enjolras: Aquarius (January 20 - February 19)
Combeferre: Virgo (August 23 - September 22)
Jean Prouvaire: Libra (September 23 - October 23)
Feuilly: Virgo (August 23 - September 22)
Joly: Sagittarius (November 23 - December 21)
Bossuet: Gemini (May 21 - June 20)
Courfeyrac: Sagittarius (November 23 - December 21)
Bahorel: Leo (July 23 - August 22)
Grantaire: currently undetermined

If I've missed anyone important, or you have reason to believe that I am wrong somewhere for whatever reason, please drop me a line :)