OK, Diina is definitely evil. Two chapters ago she secretly went back to kill Mars and she calls him a "pitiful NPC" (she knows about the game world?!) while spouting off about some sort of plan she is orchestrating. Now she reveals her deceitfulness and the ability to manipulate memories. Why do you think the golems attacked her in the tomb? Why do you think no one remembers her at first? Why is everyone's memory of her just a cardboard cutout in the background? The fact Libra and the golems don't recognize her is most damning. They are artificial lifeforms, they have perfect memories and aren't effected by illusions. Libra doesn't remember Diina because Diina wasn't there. She has isn't who she says she is and has faked her way into Lufasu's party using memory manipulation.
Question is, what's her motive? Is she a player, a god, a disgruntled NPC? The demon king? What was she really doing in Lufasu's castle when Lufasu returned? Diina must be trying to get Lufasu to recollect her military might so that she can achieve some sort of goal, but what?
I feel like she is either an NPC that became self-aware of the game world and is trying to enact revenge against the players for the war and destruction they caused OR she could just be the demon king in disguise, trying to collect the power of the former "absolute ruler" in order to overturn the balance between the demons and the humans. The demon king route seems more likely, since she has a pretty OP skillset: teleportation, memory manipulation, whatever skill she used to headshot Mars, and probably many more she has yet to reveal.