Kalhallaran was the first thought - Immune Cells already resemble The Disc in so many ways (giant sapient cells with insane interdimensionality and "antibodies") - but there's no incentive to 'experiment' on a human... but others would (like the Red Hearts guild, Ovo, By Dark, Brother Mend, etc.)...

The weird interdimensional flesh hive powering Mari's abilities in my headcanon honestly fits how Diafanics already work in my headworld, but there are SO MANY ways for Diafanics to be made that it's hard to choose. The main difference and deciding factor is Mari staying externally/visibly human.