The Chrono Phantasma (Phantoms of Time) are those who have a future that is different from the one that they originally had. Rachel Alucard is the only known Chrono Phantasma.
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Much is unknown about the Chrono Phantasma. How to become one is unknown and the specific powers that they can employ is not known either. With the rare exception, they are completely immune to Phenomenon Intervention.
By definition, the Chrono Phantasma are the ones who have had their future changed, but this also means that they have an undetermined fate. By definition, Ragna the Bloodedge, Nu-13, Mu-12, Noel Vermillion, and Tsubaki Yayoi should also fall into this category as their deaths were prevented as part of the Looping World, but there appears to be more than having a different future in order to qualify. How Rachel became a Chrono Phantasma and how her future was changed is unknown.