Maybe it's Aginor/Osan'gar re-reborn.

Apparently the messenger made the Gateway to Graendal's palace, because Moridin wasn't there and uses the TP. Moridin/SH sent Cyndane and Moghedien as messengers to Graendal once, too. Also there is no truly logical reason for the DO not to transmigrate Osan'gar, who wasn't balefired at the Cleansing. Sure, he died the second time, but at least he never was a traitor, and Moridin also states that he doesn't fear death, which indicates that the DO would transmigrate him a second time. Besides that, Osan'gar might be now in a position like Cyndane and Moghedien as Moridin's pet, maybe even mindtrapped. Considering his strength in the OP and other knowledge, he would still be useful. The messenger's beautiful body is similar to Moridin's or Cyndane's "good" bodies and it would be somewhat ironic if the old Aginor is now in the body of a youngster (but as RJ said, the DO doesn't really care what bodies the transmigrated Forsaken get). Also the additional chair at the Forsaken Garden Party in KoD could point to a transmigration of Osan'gar (maybe the transmigration delayed or Moridin sent him on some mission shortly before the meeting, before he could introduce him to the other Forsaken).

The only point that might speak against it would be that it's "lame" for RJ to resurrect one Forsaken a second time. On the other hand, it would be a twist and from the Shadow's PoV probably reasonable.

Alternatively Osan'gar might be the ugly servant..