Tenth Walker

The Tenth Walker is a type of Mary Sue found in the The Lord of the Rings fandom. It is a subspecies of the type of Sue that joins the canon heroes in their work, mission or quest. This particular Sue inserts herself (or himself, but females are more common) into the Fellowship of the Ring and travels with them. These fics tend to follow a distinct set of rules. For example, the Mary Sue almost always falls in love with at least one member of the Fellowship. It also is common for badfics, if they include the movieverse scene where the Fellowship swears to help Frodo, to have the Mary Sue swear last and include a list all of her abilities in the oath.

Fanfics in which more than one Mary Sue joins the Fellowship are less common, but certainly not unknown. Usually there is a main Mary Sue, and the others are just her followers. The largest Fellowship that has been recorded had nine Mary Sues, making it a "Code 18" Suefic.

Girl from Middle-earth
The Sue is an 'ordinary' Middle-earth girl who happens to have an extraordinary talent. She could be a great swordswoman, a healer, an archer, have psychic powers or any other ability the author deems to be cool. She joins the Fellowship and becomes indisposable to them.

There are also fanfics where the Mary Sue is the last of a forgotten race or species, such as Wood Nymph or Moon Elf. She is often a person of enormous importance in Middle-earth and is therefore invited to the Council of Elrond, where, strangely enough, no one except Elrond or Gandalf knows about her. She often has bizarre Sue-per powers that are part of being her species. Being the last of a group of people also allows her to spend most of the story obsessed with revenge and/or angsting in front of a hot Fellowship member.

Girl falls into Middle-earth
The Sue is an 'ordinary' girl from the real world who somehow falls into Middle-earth. Often she has read the books in real life or otherwise predicts events that are going to happen. She often changes key plot points such as saving Gandalf from the Balrog or stopping Boromir from dying.

Elf
The Sue is an Elf. Elven Sues are beautiful, wise, talented and graceful. They have names that are meant to be Elvish, but may be made up or taken from words used in the books. Elven Sues are noted for not being Elf-like at all, as they tend to have teenage mentalities that reflect the author's own personality.