Let’s jump right to the subject. A “Jagen” is a type of character in Fire Emblem games, named after a character from the very first game. Here is ActualLizard’s video on the subject (19 minutes), which has a lot of interesting things to say about strategy.
Jagens are characters, often given to you in the early game, sometimes available even from the very first battle. They have high stats for the early game, and are often already promoted, of the advanced classes that your other troops will have to use a special item to obtain. Jagens often have little to fear from the enemy hordes, at least in the early game.
Jagens are very useful characters, but are kind of a trap. They’re already promoted so they get few experience points from battling lesser foes, and when they do gain a level, they tend to have very low growth rates in their stats. If you over-rely on Jagens, your other characters will be underleveled, and eventually a Jagen’s slow stat gains will cause it to be unable to keep up with the increasing power of the enemies in the advanced levels. That doesn’t mean they shouldn’t be used at all, but they’re best purpose is to take the edge off of the difficulty curve and supporting your other troops. Since the original Jagen was a mentor figure to Marth and his allies, it’s an excellent case of the game’s story mirroring its design: Jagen’s days of glory are past, his true purpose to help shield and guide the next generation into becoming the best fighters they can be.But! Each game is different, and not all early game powerhouses neatly fit into the Jagen archtype. Some such characters don’t actually stay competitive for long at all, while others (like the awesome Titania in Path of Radiance ) have a strong chance of being useful for the entire game.
You should know a few things about how Fire Emblem’s character growth works. Every character has a number of stats: HP, Strength, Speed, Defense, things like that. The Fire Emblem series is defined, in terms of combat design, by its slow character growth. Every time a character gains an experience level, it only has a chance of gaining a single point in each stat. This chance is preset for each character, and gives everyone a tendency towards certain destination stats, an average spread throughout its 40 potential experience levels. What its actual stats will become will be different each game, depending on what that character rolls upon growth. While many characters have a chance of really great stats, whether they’ll achieve them differs on every playthrough. Growth rates affect that likelihood.
Fire Emblem games tend to put characters right on the edge of survival. When you go up against a boss, you may only have a handful of characters who are capable of denting its high armor, or surviving its counterattack. (Remember, in classic Fire Emblem, is a character dies, it’s gone permanently. If you want to keep using the character, you’ll have to go back to the save before the battle!) This makes it possible to get into situations where all of your characters, even if they’re of decent level, aren’t strong enough to safely defeat a boss.
Characters who join at a high level, or pre-promoted, are a solution to this. The story will sometimes hand you a new recruit to help you keep going, in the event that your party’s been betrayed by the RNG. Whether you should keep using them is something that only experience (and multiple playthoughs, or, let’s be honest, FAQs and walkthroughs) will tell you.
