Fuyumi is the daughter of Marianne, her first child born from her third husband. She was murdered while young, and then resurrected in an imperfect state by her mother and subsequently sealed away.
Backstory
In the 80s Marianne married a Japanese man, a marriage that would be the longest of her relationships. Some time during this marriage Fuyumi would be born. The exact details are unknown, but indications are that Fuyumi was deeply loved by both parents, and was likely still too young to be involved in Magical Girl business, if Marianne had intended to involve her at all.
Fuyumi would die still young, assassinated. Shortly after Marianne's victory against the Magico Lord Garak on Thrudmadra, one of his servants would follow her back to Earth to kill her child in retaliation. This servant, deeply despised by Marianne for his actions, would be cursed by her into becoming the sissy maid Giselle instead of being granted the mercy of a swift death.
In her grief, Marianne would delve into necromancy, eventually resurrecting Fuyumi. This however would not go as desired. Fuyumi's soul did not come back alone, instead being accompanied by an entity known as the Emissary of Death who did not wish to relinquish her soul. After a long stalemated fight between Marianne and the Emissary, she would eventually negotiate an exchange; a soul for a soul. Fuyumi's father would sacrifice himself for the return of his daughter. Even after this though, things still did not work out. Death had changed Fuyumi's soul and she did not come back the same. Whatever changes were wrought to her being, it necessitated Marianne sealing her daughter away, even after all of the effort and sacrifice to bring her back in the first place.
Fuyumi is currently located somewhere within the Pocket Dimension which Magical Camp is in; presumably so Marianne can keep an eye on her daughter and the seal she is under in a conveniently nearby location that is under her control.