As I mentioned in another thread, I don't think the Reaper Job's voidsent 'pet' will be literally what's left of their soul shard from the Thirteenth, but just a generic voidsent they've enthralled. It's already in the lore that voidsent can be easily bound by magic, so it's basically a non-event for Reaper to be able to do the same.
Personally I still think the WoL's Thirteenth Shard soul fragment is our certain white-robed, white-masked mage kid friend in the Rising Stones... but that's just my own theory so take that with a grain of salt.