I just found out that Rei is going to end hiatus in spring 2022, and it reminds me of a theory of mine that the space time witch mentioned at the end of volume 4 is actually Akiho (from Cardcaptor Sakura clear card). If you would like me to elaborate more on the subject I can, but the idea adds a new layer why CLAMP started clear card and stopped Rei (because it would make no sense with a new character, but if clear card is the whole backstory it would make sense).