Na had used all of her power to help Iris to go through her childhood that was nothing, but a nightmare, which made her life essence fused with her.
In another word, she was Iris. Her existence had ceased to nothingness, she was not even a spirit anymore. Na had no form nor memories, since everything belonged to Iris now and the first Serafim was only a name.
The same thing didn't happen with Lu. He couldn't cease his existence, even though the cursed lycan used Cane's body as his host, because he had promised Na to help to destroy Decratic. He was the sixth sword that Na had created, as the final attempt to bring down the devil and Lu would keep his promise even though he wouldn't be able to see Na again.
