To do:

Find if anything else remains for the unused enemy characters and items (e.g. sprites, ID assignments, ect).

Fire Emblem Gaiden is the second game in the series and the only other one for the original Famicom. While set after the events of the first game, it is not a continuation of Marth's adventures (hence the Gaiden titling), but instead focuses around two new protagonists, Alm and Cellica (each leading their own army), as they seek to bring peace to the continent of Valentia, a land divided by a war between two deities. The game adds an exploration aspect that brings it closer to more conventional RPGs, as opposed to being just a series of battles (an approach very similar to Shining Force, which was coincidentally released in Japan a week later).

It would later be remade for the Nintendo 3DS as Fire Emblem Echoes: Shadows of Valentia, this time getting an international release.

Unused Enemies

These two names appear among the list of enemy characters that starts at offset 0x3E07A .

Serpent (サーペント)

Naga (ナーガ) is the name given to the Divine Dragon King from Archanean lore. This name was also given to an unused character in the first game.

Unused Item

This appears at offset 0x3DE56 among the list of magic spells used by both, player and enemy units.

Gaia (ガイア) is a weapon that would've likely been an Earth-based magic spell.

Difficulty Select

On the title menu, set cursor to the "さいしょから" ("A New Game") option, hold Select + Start and then press A on Controller 1. When selecting a save slot, a new additional menu with a difficulty select will appear. "NORMAL" and "EASY" are the available options.

EASY mode doubles exp gain for all player units and allows unlimited trading between Alm's and Celica's groups. On NORMAL mode there are a limited amount of times items can be sent across groups. Enemy attributes and layouts are unchanged, making this mode comparable to Thracia 776's Paragon Mode, which is also unlocked in a similar manner and also granted doubled exp gain to all units.

Sound Test

This game contains exactly the same sound test mode as its predecessor, although the method to access it is much simpler: at the first game message ("1991") after startup, press Up, Down, Left, Right, Up on Controller 1. This still works in the remake, albeit at the title screen.