The moon rabbit that was named after the Watatsuki sisters' old pet, Reisen (Udonge)
Reisen as she escaped to the EarthI'm Reisen! Please allow me to be transferred here!I'll do whatever it takes!Reisen? That's such an extravagant nameFrom now on, your name shall be "Sen"
A reference to a scene in Spirited Away in which the witch Yubaba takes away the second kanji in Chihiro's name (千尋), thus renaming her as Sen (千), as a way to keep her under her control.
The 12th of September is
Reisen Day!
Apparently, it's set to this day because it's also "space day" in Japan, and Reisen comes from space (Source)
