The way I imagined it is that Julia's body and mind are kinda like universal data, and her Esper ability backs them up on the "universal hard drive". As long as there's Pure Energy, she can convert it into GPE and reconstitute herself.

I will say that if she's completely burned to ashes or atomized, she will suffer memory loss, but nothing can kill her for good.

(burning her to ashes basically kills her for a month though)