Lets boil it down to facts:
-Gunther in Fates (and thats in Early Game, he also reappears later on in Conquest as a terrible unit and in Revelations he immediately returns as a terrible unit.)
-Jeralt in Three Houses (And Jeralt doesn’t even count because he is not playable. Many like myself agree with Mekkah that he should’ve been a Jagen - I think he could’ve worked if you simply don’t let him be in the class with other units ensuring he would stay a Paladin and maybe don’t allow him to be deployed in optional missions such as Training or Sidequests (minus Gaidens imo). I actually think him being playable would’ve benifited him since then we would have opportunitys to actually get attached and give him supports with us, Leonie and the other teachers. Maybe even the lords to give Edelgard further motivation for Revenge.)
-Oifey from Holy War (He actually is a -1 so he raises the number down to 0 (or 1 if you count Jeralt) since he is a Jagen you have from the start but can only use in the second half of the game)
-Eyvel and Dagdar from Thracia 776 (These two can actually come back but especially Eyvel is a big if especially for blind players. Dagdars fate remains unknown if you miss the Gomez-Gaiden. Their deaths also occur early game and especially Dagdar can remain a fantastic unit though his Duel Skill kinda messes him up here and there)
Honorable mention
-Fernand from Sacred Echoes (Here he also doesn’t die, he just leaves the group early on and gets killed by us except if the devs changed that to him being recruitable. I am aware that this is a Fangame but it would’ve been something cool to see in the main series… WAIT A MINUTE )
-Orson from Sacred Stones (He is only playable for 1 chapter and does a good job as such a character. I think him being around longer would be for an even better betrayal. But again, he leaves the group early and gets killed later on by the players. I unironically think though that except him leaving early he is one of FEs best written characters. Simply because his simple story is executed perfectly.)
-Greil from Path of Radiance (Unlike Jeralt he isn’t even an NPC. But I think if Jeralt counts he should count too. He is a Boss that gets killed during the tutorial in training. But again that’s early and since it is training it doesn’t count. He gets killed again later on due to story reasons which is still pretty early. By the Waste EXP Logic, Greil is actually the very best Jagen since he isn’t even able to gain EXP.)
-Black Knight from Radiant Dawn (While only being playable for 3 chapters (being an NPC in one of them iirc) and truly wasting EXP he is the most OP Jagen of all time. But since you don’t get anything at all… Oh wait I dunno if his potential EXP are added to the Bonus EXP pool actually, someone who knows please give the awnser. He actually leaves the group early but returns during the final chapters of the game. He then is killed of during the endgame chapters… So he reverses things for once, nice. Interesting Ma-)
Shadow Dragons Suicide Descision (The game forces you to sacrifice either Abel, Kain, new Cavalier or Jagen to leave to die. Thats only if you play easy mode though and third Cavalier never appears if you play any other difficulty. While Jagen can technically die for story reasons here he is not the canonical choice and even if he would be, it’d be early on again. Also in New Mystery everyone who could’ve been sacrificed is magically alive again so it doesn’t count.)
Frederick from Awakening (It is implied that there are multiple different versions of not just Awakening but even the other games. Meaning that in at least some of these Frederick died due to story reasons. While it can’t be exactly determined how and when he theoretically COULD die offscreen in Lucinas original world.)
Tokyo Mirage Sessions #FE (Jagen dies in the backstory of this game just as everyone else. He is not playable but it happens in late midgame. He is a boss fight actually so idk if he should be counted.)
Ryoma from Fates (Ryoma is actually the only Prolouge character you are allowed to play with in early game under any circumstances. If you pick the Conquest Route he must be killed to progress the story. This happens in endgame so it doesn’t count.)
Mèare from Parallel Emblem (Again, a Fangame. One that is very good I heard and that is my totally unbiased opinion. Under normal circumstances it wouldn’t count but I think we can add this here because it is basically Fire Emblem Engage and I haven’t played that one yet. She is only around for two chapters and a very good unit with a weak weapon to ensure that you don’t have to waste EXP with her. But she dies off in the last prolouge chapter. While it is very tragic since in the total of 5 dialouges she has the developers managed to built up a much more fleshed out character than any of the above it still happens in early game and therefore it doesn’t count.)
In conclusion: Since we don’t know how often Fredrick died, the imaginary domain number it boils down to is U-QX-IZWBBULA-BWB
But since no one can understand that we leave Freddy out and leave at 0. Why? Because Gunther also is a reverse case since he only dies in two routes early, returns in two routes (one early and one midgame, just like Oifey does in midgame - I forgot to mention that Oifey actually leaves the group before he becomes playable.).
Actually though I think we might have to add Quan because Quan is also a prepromote that isn’t as good as Sigurd. But he fullfills many requirements. He is an excellent unit but hold back by things such as having Adapt over Pursuit making him a bit worse than others. He also gets Gae Bolg for 1 chapter before leaving and he actually dies midgame. So in reality there is 1 Jagen in the original games where we could truly say the goal of the thread has been reached.
In totally honesty I would prefer it if I have a Jagen that I can decide to train or not. If he dies because of my mistakes thats fine. But if he dies for a story reason he should die early on so people won’t get frustrated over the tragically low ammount of EXP they wasted.