Fire Emblem: Engage revolves around... well, Emblems. Echoes of heroes past given shape to protect the world from calamities. Basically Servants from the Fate serie, but with Fire Emblem characters. Most emblems are based on protagonists of previous fire emblem games, because FE Engage is an anniversary game, and it fucks mad tits.
The antagonist of FE Engage is a guy who is obsessed with one specific emblem: the Emblem of Foundations, which for reasons too long to talk about here has been lost, presumably forever. Now who the Emblem of Foundations is is never actually stated; the info we got on the guy is quite sparse, and the game keeps it vague as to who that Emblem could be.
Anyways. I think th eEmblem of Foundations is Alear, protagonist of this very game.
Evidence:
The first time the Emblem of Foundations is ever mentioned is when the antagonist almost runs into Alear, and comments: "I sensed an unexpected Emblem, and I wanted to see it for myself. [...] I had to be certain, even though I know this Emblem no longer exists." The use of the adjective unexpected implies that it's one he recognises (as opposed to, idk, "unfamiliar" or "strange"). So this guy did recognize Alear as similar to the Emblem of Foundations, if not identical.
The antagonist describes the Emblem of Foundations as (paraphrasing) "a hero who fought all alone with no ally and no aid." Now, he could just be completely wrong about it, because this game has a running theme of "people completely fucking lying/hiding stuff/rewriting history" (namely Alear's mom & Marth both actively hiding Alear's past from both Alear and History), but even if he weren't... Well, Alear in the past did fight alone- the cutscene where they kill the antagonist for the first time has someone ask them "what are you doing here all alone?" implying that while Alear did have connections & bonds with others, that specific war effort was them on their lonesome, which is consistant with how the antagonist describes the Emblem of Foundations
The antagonist tells Alear by saying "I will forget you. Even if we meet again, I would not recall your face." Which, cool taunt, but also that guy tends to be very literal/genuine in his speech. I absolutely believe he wouldn't recognize Alear- as the protag, or as the Emblem he's looking for. Alear even responds with "so you'll make the same mistakes as before."
The summon prayer for the Emblem of Foundations is "Burn, Emblem of Foundations." Alear is literally the Fire Emblem.
At the very end of the game, the antagonist calls forth the Emblem of Foundations, and then acts glad to be reunited with them again, despite the fact that no one showed up. Characters speculate that he might be hallucinating, or seeing something only he can see. I don't think so, however. I think we're seeing exactly what he's seeing- Alear.
And, obviously: choosing to read the Emblem of Foundations as Alear just makes this whole game absolutely delicious. Alear tells the antagonist multiple times "bro if your issue was loneliness why did you obsess over this emblem instead of reaching out to me?" And in the case where Alear is said emblem, then we've got a wonderful tragedy at hand. The antagonist failed because he was so obsessed by a far-off chimera he couldn't see what was literally in front of his eyes. He was doomed from the start. His dream was at hand's reach, but he couldn't see it. He couldn't see a single thing.