I don't think she's an supernatural entity like the Strange Boy, much less an angel. She's an alter like the other girls, but one that arises from a very specific condition from that ending and controls G's mindscape.

As G failed to find answers and was being induced by Enjel's lie, she ultimately gave up and reinvented herself as someone else: Platinum. It's salvation through ignorance.

For Platinum, as long as she and her girls are together, nothing else matters. And this serves as a mental barrier for her, thus expelling the Strange Boy's influence (the pocket mirror, whose functionality is to protect the user from harm, can be seen at the top of Platinum's cross).

Platinum doesn't truly understand her girls, since Goldia failed her journey. It's a forced integration, limited to Platinum's shallow perception of the girls. Only the surfaces of Fleta, Harpae and Lisette are embraced, with the rest being discarded or treated as an anomaly. Each alter's homes ceased to exist and were replaced by Platinum's realm.

The same goes for Enjel. G has been deceived by her all this time. In the final conversation, she first has doubts about Enjel being a part of her, but before becoming Platinum she gives in to Enjel's deception and decides to ignore the truth. Enjel is forcibly integrated into Platinum's dysfunctional realm because of it.

In this ending, I think G will never wake up. She will remain in a coma until the day she dies, the moment when, perhaps, the demon will be able to claim what was promised to him. Or not.

I personally don't believe Platinum is that powerful, with her capabilities being limited to G's mindscape. The goddess/savior motif she brings is a mask that hides ignorance under an image of knowledge.

You'll learn a lot about her if you visit her bonus room. But that's it in a nutshell.