what would a Sylph of Void be able to do? Repairing nothingness, Repairing voids, so like... healing physical wounds? Healing gaps in knowledge or memory? Creating and harnessing a blackout like we see with Roxy or Rose?

Think bigger. They could definitely do that, but remember Roxy? She Removes the Lack. Someone who Improves the Lack and someone who Makes the Lack Better sounds like they could ambivalently make things disappear and appear at will. May want to stick with making things Disappear if you don’t want to have too much overlap with Roxy. Other than that though? Remember Void and Horrorterrors go hand in hand too. Healing through Dark Magics of the Horrorterrors could be fair game as well. Aside from that, they could likely, indeed, harness the Blackouts or rather, be able to obscure just about any piece of information. Oh hey, who was asking before for a Classpect able to make people forget your very existence? A Maid or Sylph of Void may be able to just erase every trace of you ever existing.