It's Hydaelyn (Venat).

Quote Originally Posted by Absimiliard View Post
[Cosmos'] death wound up being permanent. Part of her essence returned as Materia in Dissidia NT, but this incarnation has no recollection of its former life and very little of its previous strength.
Well yes, but actually no.

Cosmos returns from the brink after her Warriors' victory in cycle 13 (Dissidia), but Shinryu traps her in a nightmare world along with Cid of the Lufaine where Chaos keeps winning until he goes insane from amassing too much power; a group of heroes arrive to put a stop to it by defeating Feral Chaos (Scenario 000 a.k.a. Confessions of the Creator, the postgame of Duodecim). Cosmos is still alive along with Cid in World B following the story's close, but given it's bound to collapse without the cycles to maintain it, she's probably still dead.