MC also said that there is something she is still hiding, and he is correct. Look back at the story and you'll realize there is a contradiction. Elmia is supposed to be unkillable, and was sealed inside that coffin because of it. However, if you look at her now, she is clearly a ghost. That means she died when it was clearly said she couldn't. That is a huge contradiction. This also falls into the factor that she now needs a body to become her own. This would be void if it was true that she cannot die in the first place, as she would not be a ghost but her own original self at that point. The unfortunate thing here is that MC will now have to rely on an ass-pull by the author to get out of his predicament. There is clearly no way he can beat her if she can just create anything against MC during battle. And it would be really cheap if MC suddenly just awakens some power that can defeat her for plot purposes.

She never said she died though, only that she's "close to being" a ghost, and that her soul is anchored to the coffin. If she can't get her body out, the next best thing would be to procure new body. Well, there's also the question of why her soul looks like monster.

Checked back, she also said the sealing spell is something she left for her people to seal her.

This is probably the thing where typical harem protagonist's problem can become an advantage. He need to choose between the four girls. Harem protagonist can't do that before ending (or the shipper of other girls would rally to the author with pitchforks) :D