Mary Sue

Mary Sue is a term used by Incubators to describe girls from a dimension where magic is non-existent who become Magical Girls to enter a realm that their world considers fictional. One document that provides crucial information about them is the "Mary Sue Encyclopedia", dedicated to documenting all Mary Sues from 1991 onwards.

Nature
The typical Mary Sue's journey goes like this. When a girl makes a contract with an Incubator, her Soul Gem is sent to the dimension of her choosing where she proceeds to create a new body to walk around the world. There she acts like just another Magical Girl until becoming a Witch or being killed in action. However after her defeat any trace of her is erased from that universe's existence as not to disturb its natural flow, the only exception being if another Mary Sue takes her Witch's Grief Seed.

History
While wishing to travel into fictional worlds wasn't anything new, the term didn't come into existence until 1991

when a young Japanese girl named Meiri Suzuki made a contract with Kyubey to become a Sailor Senshi, only to be killed by Queen Beryl later on. However Mary Sues eventually faded out after being outlawed by the Law of Cycles.

Known Mary Sues

 * Meiri Suzuki
 * Ebony Dark'ness Dementia Raven Way
 * Michi Hitachiin (Implied)
 * Niji Akemi
 * Etsuko Ajibana (Implied)

Trivia

 * They are based off of "Mary Sues ", a term in fan fiction that desrcibes a poorly written characther so perfect they are not even challenged by the events of the narrative, and are often self-inserts for the author.
 * The Runes on the cover literally translate to "The Marysue Encyclopaedia".
 * While the Enyclopedia is mostly written in Runes, there are bilingual versions such as the copy Homura found  that exist as reference material for Magical Girls.
 * Two episodes of SiRenfield's "In a Nutshell" on My Immortal and Michi Hitachiin contain subliminal messages written in Runes that say that Ebony and Michi were Magical Girls. The former even has an indirect mention in the epilouge
 * Coincidentally there's a Wild Mass Guess on My Immortal's TV Tropes page that speculates that Tara (the writer) did make a contract with Kyubey.