Corrin

The Avatar (マイユニット, Mai yunitto, My Unit in the Japanese version), default name Kamui in the Japanese version, is a playable character in Fire Emblem if and the main protagonist of the game. Like in Fire Emblem: Shin Monshō no Nazo ~Hikari to Kage no Eiyū~ and Fire Emblem: Awakening, the Avatar's identity is customizable by the player.

Profile
The Avatar was born to the Hoshido royal family but was raised by the royal family of the neighboring kingdom of Nohr. A great war looms over Hoshido and Nohr and soon the two kingdom meet on the battlefield. The Avatar must choose to side with a kingdom and help with their respective causes.

Hoshido
If the Avatar sides with Hoshido, they will be defending it from the invading Nohr kingdom

Nohr
If the Avatar sides with Nohr, they revolutionize the Nohr kingdom from the inside.

Etymology
In Hindu Mythology, an Avatar is a terrestrial manifestation of a deity. The term is commonly used to refer to an entity that represents the player in a video game.

Kamui is the word for a divine being in Ainu mythology, although it is almost never written with kanji in modern times. In a more general Japanese context, Kamui refers to the might and majesty of the gods, particularly the Shintō Kami. This is more to tie the Avatar's roots to the Hoshido kingdom which is based on feudal Japan