Who is this interestingly named person, mentioned in Tosafos?
A. A. Orbach, in "BA'ALEI HA'TOSFOS," writes (Sanhedrin, p. 509 fn. 61) that he was not able to determine who the Nakdan was.
There was one Berechyah [ben Natrunai] ha'Nakdan of France, who authored the famous "MISHLEI SHU'ALIM" and some other philosophical works (SEFER HA'CHIBUR, SEFER HA'METZAREF), and was a contemporary of the Roke'ach, Rav Shmuel of Ivra and the Ohr Zaru'a.
Another potential candidate might be Rav Yosef ben Yehudah ha'Medakdek (Rav Yosef Chazan of Troyes) who lived during the same period and authored a work (which has not reached us) called "Sefer Yedidus" on Dikduk.