Who are the main characters of Fire Emblem?

It really depends on which fire emblem your talking about.

If you want the main character, they're usually the ones of the 'Lord' class.

For example,

In the first Fire Emblem game and its sequel, Marth is the main character. There is also a large cast involved in every Fire Emblem game.

The first Fire Emblem to be released outside of Japan (Fire Emblem 7: Blazing SWords) had three lords; Lyn, Hector and Eliwood (the latter being the father of Roy, the protagonist of the game's sequel, which was never localised.)

In Fire Emblem 8: Sacred Stones, the lords are Eirika and Ephraim.

In 9 and 10: Path of Radiance and Radiant Dawn, respectively, Ike is the protagonist despite not starting out or finishing as a lord, and Micaiah is also an important part of the latter game.

Other than the lords, I previously mentioned there being a rather large cast of playable characters in each and every game.