At current world, Pure Energy is everywhere just like air. This is caused by the magical calamity incident. As for why, unfortunately I can't explain it now, it's part of the plot in this story. But please note, that it doesn't close the possibility of someone creating an "anti-Pure Energy" room that prevent Pure Energy to enter/exist inside.

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Important to note that Esper can only use their [GPE] to manifest their power, they cannot use Pure Energy like a Madou used by Mage would. In normal case, Julia use her [GPE] to preserve her body on its "Prime State" which is around age of 25 judging from her outer appearance. But from there on, Julia Miller's power can go in two directions:

1. She constantly renew backup data of her current self (including memories and body) in her mind, either automatically or manually. By the the time her body is close to destruction (by enemy attack or whatever) she will execute the command "Create a new body and transfer my mind (consciousness) to it" and her power will use all of her remaining [GPE] to replicate her body. In the middle of this, her mind will be flying in the air until it goes to the new body. If this happen, Julia Miller will undoubtedly becomes younger (starting age of 7 to teenager) since her [GPE] will not be sufficient to fully reconstruct her body. She will suffer minor memory lost as well.

2. Part of her power is "Pure Energy Manipulation", but in case of this, she can only manipulate pure energy to reconstruct her body, nothing else. Why she can only manipulate that far is because that much is her mind can handle. Esper's genetic predetermined how far she can do, and it's decided by birth, cannot be changed. So, in case her body is close to destruction (like realllly close, say that she only have her head left) her mind will execute the command "Manipulate the surrounding Pure Energy, reconstruct my body, and transfer my mind to it". But the downside of this, even though Julia Miller is back to life, she's not exactly the same person she used to be. Body created by [GPE] posses genetic data which required to full reconstruction, but since she use Pure Energy for it, she can only create "A vessel that is like her", a mere copy you can say. The mind that is transferred to this new body will eventually dissolve itself to fit the new body, so, it's no longer the same Julia Miller.

If you chose 1: Julia Miller can be truly die if she killed over and over again until she no longer posses enough [GPE] to reproduce her body. She won't have enough [GPE] to make anything if her physical age is less than 7 or so, it can't go any lower than that, or she can get really desperate and revert herself as a baby.

If you choice 2: She can't die unless she wanted to. Unless being put into "Anti-Pure Energy" room, if any.