Jagen

Jagen (called Jeigan in the Japanese versions) is a Veteran knight of Altea. He acts as Marth's guardian. He is an invaluable unit at the start of the game due to being a much higher level than the rest of your units, but he has terrible growth rates and potential that severely handicap his usefulness later in the game. He steps out off the battlefield in Fire Emblem: Monshō no Nazo but still stays by Marth's side as an advisor. Afterwords, he dies and his position is given to Cain.

Archetype
The word "Jeigan" is also used as a term for characters which use up too much exp at the start of the game (only if you choose to use them) and so hinders the other characters growths.

Jeigan's are usually promoted Paladins that you get in the first chapter, and appear to be amazing units due to having much higher stats than your other units. However, due to being promoted, a Jeigan will usually gain very little experience for quite some time into the game, the original Jeigan being an exception. Also, almost all Jeigans have terrible growth rates and barely any endgame potential. Recent Fire Emblem games are abandoning this trend however, with the Jeigans in Fire Emblem: Rekka no Ken, Fire Emblem: The Sacred Stones, and Fire Emblem: Path of Radiance (Marcus, Seth, and Titania respectively) all ending up as decent units that are worthy of using for a long time, the latter two remaining great into the endgame.

Fire Emblem: Radiant Dawn, the most recent Fire Emblem game, has no Jeigans whatsoever. There are however multiple Oifeys: Tauroneo, Volug, and the Black Knight, and partly, Sothe and Zihark. The Jeigan from Fire Emblem: Path of Radiance, Titania, makes a reappearance, but not until late in the game, and she is arguably the best Paladin.

List of Jeigans
There are others who are considered Jeigans, but this is the generally agreed upon list.
 * Jagen (FE 1/FE 3 Book 1)
 * Arran (FE3 Book 2)
 * Evayl (FE 5)
 * Dagda (FE 5)
 * Marcus (FE 6/FE 7)