Phyre was once a member of Marianne's Magical Girl team, and a prominent researcher into anti-magico weaponry and techniques. Her current whereabouts are complicated unknown.
History
While Phyre is never encountered in-game, she is brought up in conversation and written records quite often. Born on a magic-rich world, Phyre was an accomplished spellcaster even before she signed a fairy contract and became a proper Magical Girl. At some point, she proved herself powerful enough to earn a place on Marianne's team, and became a major influence at Magical Camp, working tirelessly to improve humanity's odds of defeating Arcanos.
Her personal life was far less heroic. Although once sane and reasonable, Phyre realized at some point that her power was limited by her sanity. She solved this limitation the way any reasonable thaumaturge would: by diving into research of the Subconceptual Plane, a realm in which the very laws of nature are twisted and shattered. The experience changed her instantly and irrevocably.
Whatever moral constraints once held Phyre back from unethical research projects were gone, and she began performing experiments on Magical Girl trainees and regular humans with little regard for their wishes or well-being. Isabelle, Marianne, and possibly Yoshida were all unsuspecting recipients of Phyre's efforts, with varying degrees of success. Additionally, Phyre's personal tastes began to express themselves prominently in her work, particularly her interest in futanari. While some of this behavior was harmless and temporary, Isabelle in particular suffered lasting behavioral harm as a result of Phyre's experiments.
Eventually, Marianne decided that Phyre's madness was too great a threat to be allowed to continue unimpeded. The mad sorceress was imprisoned and her research laboratory was sealed shut, but the exact nature of her imprisonment is very uncertain. Wendy and Shelly are both characters who claim that they are, or were, or are in part, Phyre. While it's certain that there is a connection between these three figures, the details are still largely unknown.
Additionally, Phyre enjoyed reading extraordinarily bad speculative fiction novels. Whether this was a result of her exposure to the insanity at the heart of the Subconceptual Plane or merely a sign of her bad taste in literature is something the world shall never know.