That's a designation I use for her, not a username or her real name. It's just for convenience with online friends/ online people I talk to. It's usually based in a facet of the individual's personality, their career, or something related to them. For example my most recent ex was "Jaguar", the girl before her was "Nurse" (guess what she did for a living), my ex-wife was WoWcrack, and then later became "Satan".
Ceres is the Roman name for Demeter, Ceres protected transitions of women from girlhood to womanhood, from unmarried to married life and motherhood. So there's me being a cheeky mythology nerd there. It also has a slightly verbal dual meaning because it's just a little off of the name of the character she was cosplaying as when I met her.
Seras ^^
Why yes I do think I'm quite clever. Thank you!