Word of God: According to Koei Techmo themselves, they were left in the dark about that other Fire Emblem games were in development at the time.
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The development team was planning to release a piece of free DLC where you could rescue Darios from their Plotline Death and unlock them as a playable character, but there was no room to add them.
Very little behind that is known about it. For unknown reasons, this idea was altered into the Arena style maps instead.
There originally was a mode called "Coliseum Mode", where the idea was the player would take a few heroes into an arena and fight.
Gaius and Lon'qu were considered for the Awakening-themed DLC pack, but Olivia was chosen instead due to the lack of daggers for Gaius and Lon'qu being conceptually similar to Lyn and Navarre.
Data in the game suggests that Cordelia's personal weapon was supposed to be Gungnir, but it was replaced by an original weapon, Cordelia's Spear.
Cordelia has a cut support conversation with Camilla in which Cordelia expresses jealousy over the latter's more feminine body.
Niles was supposed to have Bond conversations with Sakura and Lianna, but, for whatever reasons, they weren't used when Niles was made available with the Fates DLC pack.
She would later be added as part of the Fates-themed DLC pack.
Azura was initially planned to be in the base game, but was cut early on to avoid over-representation of Fates characters.
Sumia was considered over Cordelia as the Pegasus Knight representative from Awakening, but Cordelia won out due to being more popular.
A talk with Nintendo and Intelligent Systems changed their minds (Nintendo/IS likely reminded them that Lucina is massively popular.)
Lucina was dangerously close to getting axed early in development due to being a sword user.
The dev team didn't want the experience to change based on which character was picked, and they also wanted to establish how the weapon triangle worked starting with the sword.
Rowan and Lianna were originally intended to use axes and spears, respectively, but were changed to sword users for the sake of the tutorial.
Unlike the English examples, there's no Bond conversation between Navarre and Niles.
It's not limited to just English, Takehito Koyasu provides voices for Navarre, Validar, and Niles in Japanese.
The Shadow Dragon DLC pack gives us a Bond conversation between Hinoka and Minerva, both voiced by Cindy Robinson.
Takumi and Niles, both voiced by Michael Sinterniklaas, are given a Bond conversation in the Fates DLC pack.
Because of this, he employs a bit of Vocal Evolution for Ryoma, whose original portrayal was quite similar to Chrom's. Additionally, the two have a Bond conversation together.
Both Chrom and Ryoma are voiced by Matthew Mercer in English.
Role Reprise: Everyone's voices from Fire Emblem Heroes return in Warriors, which means that characters who were hit with The Other Darrin in Heroes keep their new replacement.
This is also likely the reason why Validar makes the most amount of appearances out of any character in History Mode. Most notably, Lissa takes Elise's place in one map while the latter never appears in a single History Mode map.
Real Life Writes the Plot: Some characters do not appear in DLC History Mode maps ever, implying their voice actors were busy or budget cuts.
Lyn: Zhenji, Daqiao, Xiaoqiao, and Xingcai.
Frederick and Validar: Xu Huang, Zhou Tai, Ding Feng, and Sima Yi.
Ryoma and Chrom: Taishi Ci and Jia Chong.
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Relationship Voice Actor: Quite a few English cast members from the game were also involved in parent series Dynasty Warriors, particularly circa games 5-8.
Real-Life Relative: Twins Rowan and Lianna are voiced in Japanese by siblings YĆ«ma Uchida and Maaya Uchida, respectively.
This would later carry over to his eventual appearance in Heroes.
The Other Darrin: Iago, as the only character from Fire Emblem Fates to not have been in Fire Emblem Heroes at the time, has his voice actor changed from Walden James to Kirk Thornton.