I'm not sure of anything anymore, but I do think that Diina, or whoever she really is, really is involved with the MC's soul being summoned to the game world. And I think that summoning the MC's soul to the game world was probably the only way to bring Lufasu back, for whatever reason. And people not remembering Diina, or Aigokeros suddenly remembering her, is probably intentional, she is probably manipulating their memories, for whatever reason. And I don't think she is a traitor, I think everything she is doing is probably related to, either some mission the real Lufasu gave her, or part of a plan to bring the real Lufasu back, or both.