Chris Avellone, Lead Designer of Torment (source: comp.sys.ibm.pc.games.rpg)

We intended that the Nameless One's real, true, first name would never be revealed in the game, since anyone playing the game could imagine a much better name than any dumb name we could come up with. While it is possible that the NO was once named Yemeth during one of his incarnations, Yemeth is not his real, true, first name. Hope this helps, Chris Avellone

Chris Avellone (source: Interplay forum)

The Nameless One is not Zerthimon, and the Nameless One's "true name" is never revealed in the game - even I don't know who he truly is. :D Hope this helps, Chris

Dave Maldonado, another Designer for Torment (source: Interplay forum)

I don't know TNO's true name; as I understand it (and as Chris stated), it doesn't exist in any official capacity (and as such it certainly isn't Adahn, Yemeth, etc.). Really. -- Dave