Self-Insert

A Self-Insert, abbreviated to SI is an original character meant to represent the author, or are based off of living people.

While closely related to Mary Sue, an SI will not necessarily be a Mary Sue, while not all Mary Sues are necessarily SIs.

They can range from being a relatively reasonable original character to being an idealized Mary Sue version of the author, or even occasionally an Avatar. Often, SIs in Humorfic are self aware of their status as a SI character.

=SIs in Fanfiction=

Self-Inserts in fanfiction often include Trans-Dimensional Snatching or Hopping, depending on how ordinary or powerful the SI is.

Occasionally, the Self-Insert will have been made that they live in the canon universe originally, rather than relying solely on universe-hopping. This is usually more common in canon universes that are modern (or modern-like). Thus you are more liable to see an SI who has lived in Metropolis or Tokyo their whole life rather than one who has was born in Bree or Mossflower Woods. This does not always prevent the SI from kidnapping the canon characters, however.

Many Self-Inserts in Fanfiction are often used in humor fics so that the author can pretend to hang out with their favorite character or characters. This often breaks the Fourth Wall to varying levels. How the characters feel and react to this varies with how out of character they are and how reality bending the SI is. The reaction of canon characters often ranges the full gamut from slavish obedience and adoration to full-out hate for the SI in question. One very common setting for SI's is the Subreality Cafe where the much abused characters can hang out and have a few drinks. SI's appear in the form of authors, and their Muses. Several Fandoms have their own version of the Subreality Cafe, one of the busiest is Doctor Who

Rarer than the Self-Insert for humor fics are SIs who are in seriously (noncanon breaking) written fanfic where the SI fully participates in the world.

=SIs in the PPC=

Many Agents in the PPC can be considered incarnations of their author counterpart and thus Self-Inserts. However, anyone who suggests this to some Agents might be spending some time in Medical for "Self Inflicted" injuries.