Enemy Units refer to the units that the CPU (or multiplayer opponent) controls directly.
Enemy Units can come in a variety of classes, but depending on the story chapter, enemies may come in a certain theme. Most early game Enemy units are bandits groups consisting normally of Brigands, Barbarians, Thieves, and occasionally Mercenaries. Mid to late games usually revolve around facing an enemy army of another nation, adding many typical classes associated with them including Knights, Cavaliers, and Mages.
Enemy Units are usually represented by the color red; a red-colored HP bar, a red circle surrounding the unit, or red-colored clothing/hair. This is also generally the color scheme of that army's nation. Fire Emblem Fates had a unique exception for one enemy faction that had purple sprite units. Fire Emblem Warriors is the first game to begin the special occasion of having two separate enemy armies that fight the players and each other. One army is red, like always, but the other has yellow units. The yellow units are normally labeled as a "third army." Older games did not give multiple factions the ability to fight each other even though they were enemies in the story. They could only fight the players when the level started.
Enemy Units tend to have relatively average stats and wield weapons generally equal or lower than the current army's weapon spread. This is all meant to fit the general progression of the player's army by that chapter, providing a decent challenge to the player. Enemy units either rely on sheer numbers, but mediocre stats to overwhelm the Player's army or competent combat prowess in exchange for numbers. Either case is meant to test the player's tactical skill with unit management. On occasion, enemy units may drop weapons and items when defeated, helping to keep the player's army supplied or giving them new weapons to deal with specific unit types introduced in that chapter.
Enemy Units use generic sprites/character models for a vast majority of their army, making all individuals sharing the same appearance have the exact same appearances with each other. The lone exception to this are boss characters who not only have unique character portraits, but often unique battle sprites or combat models. These units are generally higher in power when compared to the generic enemy units scattered throughout the field.
Certain Enemy Units can be recruited into the player's army, usually by having a specific unit, usually a Lord or an ally with a relationship with that unit. Some require more unique requirements to be fulfilled. As potential playable characters, these Enemy characters have more unique stats and are oddly competent enemies. Recruitable Enemy Units are usually noted by their unique map sprite, character portrait, and having an actual name instead of a generic title.
Majority of generic enemy units are male in appearance, even for classes that have unisex playable characters of that class such as Mage and Cavalier. Female exclusive classes like Pegasus Knights and Valkyries are the lone exception as they are female exclusive classes normally. The main exception to this is the fourth and fifth game where generic enemy units such as Heroes, Mage Fighters, and Wyvern Riders would sometimes be female. Fire Emblem: Three Houses also shares this distinction, with enemies in story and paralogue chapters sometimes being female. This is not random, as the actually random auxiliary battles never have female opponents. Female variants of every class that has them do appear, although the only female Dark Knight appears in Azure Moon chapter 21.