ALEXANDER SIDDIG as Advocate XYLAS in FOUNDATION 1x01 The Emperor's Peace
Per Deadline, Foundation season 3 has added a new series regular. Alexander Siddig, who starred on Star Trek: Deep Space Nine as Doctor Julian Bashir, will be taking on the role of Dr. Ebling Mis, a psychohistorian with a keen interest in the work of the show's main character Hari Seldon (Jared Harris). Siddig previously appeared in the premiere episode of season 1 as Advocate Xylas.
It remains to be seen if the two characters played by Siddig have any sort of in-universe connection. However, at first glance, it seems that Advocate Xylas would be entirely disconnected from Dr. Ebling Mis. In the show's premiere episode, Xylas appeared as part of the tribunal trying Hari Seldon. The character was staunchly anti-psychohistory and held the opinion that Seldon's studies were meant to be a destabilizing force. [x]