First of all, Gaiden was still Akanea despite being a different continent and last time I checked SerenesForest listed SS as Elibe
Sacred Stones isn't Elibe.
Gaiden is explicitly stated in the same world as the Akaneia series. Different continent, but same setting. Gaiden means side-story in Japanese, which is exactly what this game is. Even some characters from FE1-3 appear in this game, such as the Pegasus Sisters Est, Palla and Catria.
In fact, Awakening serves as a distant sequel to the Akaneia series and features both Akaneia and Valentia, albeit renamed as Ylisse and Valm.
Sacred Stones is a different world altogether with no connections to Elibe and has a different backstory altogether. Some people classify SS as Elibe because it's to have all 3 GBA games under one category, but it isn't canonically part of the Elibe saga.
Pretty much.
represents the GBA similar to
representing the 3DS/Modern era.
You can say that, yeah.
Roy was the first GBA era protagonist, much like how Ike was the first GCN/Wii era protagonist.
You could argue that all FE characters represent different eras and changes in the franchise.
Marth could represent the NES/SNES era as it was when the series was handled by the original creator, Shouzou Kaga, and they all have a similar art style and presentation. He also has the DS remakes and was redesigned this time to match his latest appearances.
Roy's game was sort of a reboot of the series after Kaga's departure from IntSys and the first handheld game in the series. It simplified some mechanics and, along with Marth, was influential in bringing the series to the West thanks to Smash Bros. Melee.
Ike's games saw a return of the series to home consoles and the first full saga to be released outside of Japan, as well as being the first 3D games of the series.
Robin and Lucina come from Awakening, a major success in the series and essentially saved the franchise, as admitted by IntSys. The 3DS game also saw many changes to the games, bringing a new art style and new features.
And fanservice.
Fire Emblem is just extremely lucky, honestly. It has nothing to do with representing anything. The series was only going to have 3, and would've been set with that.
Marth was the original and most popular, the Pikachu. Ike was added as a fully unique newcomer for the series in Brawl, and none of those got cut (disregarding legal and tech reasons). Robin was added because Awakening sold well, and he was quite unique and could represent Fire Emblem well overall.
Lucina was only going to be the equivalent of Alph or Male Wii Fit Trainer. Roy only got in because he was next in line as a Melee rep once Mewtwo got in. If Dr. Mario hadn't made the base game, it's possible he could've been picked as well (I still think it would've been Roy, but who knows). He probably would not have gotten in as DLC of the set wasn't planned as Brawl, Melee and Newcomer.
That said, since the series does have 5 now, and the only one who was even considered for a spot was Chrom and he has a fairly large in-game role that it wouldn't make sense and would be a lot of unnecessary work to cover. I doubt any FE character would've even been considered for DLC if they didn't do cut characters.
Pretty much this.
Marth, Ike and Robin were the only planned FE characters. Lucina and Roy were added out of different circumstances.
Roy only is in the game thanks to his popularity mostly stemming from his previous appearance in Melee. He was also considered for Brawl, but this time around, it's likely that he wasn't as considered as before with Awakening being the new big thing, so obviously Sakurai would prioritize someone from that game.
Still, I think Roy as a somewhat important character thanks to his role in bringing the series to the West along with Marth. I don't think Marth alone would've attracted more attention. Roy's flashy fiery style helped things, setting him apart from Marth and showing the character diversity that exists in the FE series.
