That's an interesting question. I don't think it's mandatory, per se, that a player selects Saraneth. There are a few options worth considering:
- As you mentioned, the GM can run Saraneth as an NPC or GM-PC
- The players can control Saraneth as a group
- The players can take turns controlling Saraneth
- You can actually run the game without Saraneth! Any information, power, or authority that she has could be gifted or granted to one of the other party members. Perhaps she gives someone her magic staff, or she tells someone King Nothing's secret name. It can be really fun to figure out, as a GM or a group, why Saraneth isn't there (is she dead? trapped? diverting KN's attention elsewhere?) and how she--or the party!--has made up for her absence.
Of course, please feel free to add whatever guidance you think is helpful in your translation :)