The most tragic character in the franchise?

Reinhardt, served his country and upheld his duty, but only wanted to reunite with his sister and be with his love Ishtar in the end.

Most tragic? I dunno, but there's Lyon.

1.Nyx from Fates had a pretty tragic backstory. The whole cursed to be found thing, and how she will never brake the curse.
2.Sonya from Fe Sov is also pretty tragic with her sisters being dumb faithful and all.
3.Although I don't know much about her Nyna from Martha's game was also pretty tragic, I think. She fell in love with Zeke/Serious/Camus and he left her for Tatiana💔 within good reason.

Lyon by far. The poor dude was overshadowed and was depressed all his life. Ends up losing his dad, is pushed into being emperor and gives into depression and the Demon King. He's fed the hope of saving the entire continent if he gives his soul away only to find out that it was futile, fading from existence only to lose any and everything. His admiration for Ephraim and his love of Eirika, all lost in the Deep to never be seen again.

I don't know much outside FEH, but Laegjarn seems like a reasonable candidate. Her father used both his daughters and evidences not the slightest regret that she fell into Askrian hands. We have, unsurprisingly, no mention of the mother. She seems to have a strong sense of justice, according to the way she treats the colonized region of Nifl. She cannot defect to Askr, seemingly because she does not believe them capable of overcoming Surtr, but perhaps also because some part of her is still desperate for some sense of family-based identity. For all her justice and her ultimate sacrifice for her sister, who was likely her only real sense of identity and a weak one at that, she received nothing. She expired, consumed by the fire dragon, never to see Askr's victory against Surtr, which could have begun a process of freedom for her, or even the rescue of her sister. From the beginning to the end she was empty, and those present at her death did not even have the wisdom to console her with a promise that she would be remembered and mourned for her virtues.

I would say Pelleas from Radiant Dawn is a tragic character, if you discount the second playthrough. Since the beginning of Act 1 he stands in the shadow of Micaiah, because she leads the Dawn Brigade and conquers the Capital in the end. His Backstory is not much better. He was raised in a orphanage until Izuka found him and told him he was the son of King Ashnard. In the end this was not true and he lifed a lie. The worst thing about his story is without a doubt, that he was used by the Senate of Begnion by signing the Blood Contract (one of the most controversial things in Fire Emblem). To make it short, in Act 3 he dies by trying to protect his people from the Blood Contract by sacrificing his own life, but this fails.

I could talk hours about Pelleas, because he is my favorite Fire Emblem Character to this day. His Backstory is really interesting and overall the Story surrounding him. (Sry for my not perfect English, it is not my mother language)

Pretty much anyone from Genealogy could fit, especially first generation, but I’m going to go with Brigid/Eyvel. She’s kidnapped by pirates, her crew mutinies against her, she joins Sigurd’s army, she loses her memory, her adopted daughter is kidnapped, possessed, and forced to fight against her, she’s petrified, and when Finn finally tells her about her past at almost the end of the game, she doesn’t believe him.

I'm gonna say Finn from FE4/5 is a strong candidate. He was only 15 years old in the first generation, when his lord, friend - and possibly surrogate family - died. He had to flee his country, look after Nana and Leif constantly while being on the run. The last order he got from his late lord - to protect Leif with his life - was so deeply rooted in his mind that after this day, Leif stated that he didn't see Finn laugh or cry once. He was so focused on his mission that he did not care about anything else anymore.
Perhaps the saddest thing is his ending in Fire Emblem 5, where he suddenly just disappears for three years without anyone knowing where he went, though he has been seen in the Yied Desert, the place where his lord died. So he most likely spent three years scouting through the Desert because he still hadn't lost hope to see his lord alive, even after twenty years.

tl,dr: Finn is an amazing character and a tragic one as well. Tailtiu also fits the bill...as does almost every Jugdral character.

Lyon of course.
Like someone said before, he lost all he had, and all he wanted was to save his country from a natural disaster.
-He was always so underrated
-He lost his father and killed their best friends father...
-He was corrupted by the Demon King, and was used like an slave...
-He was killed by their best frieds...
-He, after all, got exactly the last thing he wanted: destroying and devasting his own country.