A Mary Sue is a character, typically female (though, male counterparts, named either Marty Stus or Gary Stus, exist), who is unrealistically favored by the story she exists in, by being either the center of attention, unable to be harmed and/or instantly making her favorite characters fall in love with her.
The adjective used to describe a character as a Mary Sue is "Suetiful", coming from the name of a TV Tropes trope, Suetiful All Along.
The two most prominent Mary Sues of MarissaTheWriter fanfics are Marrissa Roberts and Gloria Deschanel. Each of them showcases many Mary Sue traits, mainly as a parody of fanfics of such kind, rather than a genuine example. For example, Marrissa and Wheatly have had a romance ongoing without any clear setup, Marrissa has speshul powers, whenever things get moot for her she always manages to fix them, despite claiming that she cannot, and Gloria is able to achieve multiple unrelated things for the protagonists' good at once.
MarissaTheWriter, on many occasions, denies that Marrissa and Gloria are Mary Sues. Chapter 7 of ITS MY LIFE! ended with Marrissa being on bad ground, in Chapter 12 Marrissa actually died, and Gloria is reported as a not-Mary Sue because she is not based on MarissaTheWriter's character and is a Christian.
Ebony Dark'ness Dementia Raven Way, Marrissa Picard and Kobito Minano, Mary Sues from other writers' stories (My Immortal, Stephen Ratliff's stories and Tarvos and Fairie: A Love Story respectively), appear in MarissaTheWriter stories. They do not share the paradigms of Marissa's Mary Sues, instead being introduced as the "worthy" adversaries to the native Mary Sues.