Ally Units (Sometimes Allied Units) are units who appear in Fire Emblem: Genealogy of the Holy War, Fire Emblem: Path of Radiance, and Fire Emblem: Radiant Dawn. These are not to be mistaken with Player Units, as these units are controlled by the computer rather than the player. Ally Units will fight the Enemy Units alongside the Player Units. Ally Units are usually represented by the color yellow; A yellow health bar, a yellow ring around them, and tinted yellow clothes are present with all Ally Units except in Genealogy of the Holy War where they are green colored. Ally Units move during the Ally Phase, which occurs directly after the Player Phase.
Ally Units are often generic units (e.g.: Crimea, Daein), although sometimes they are notable characters. In Tellius Series although these units are not directly controlled by the player, they can be given commands, like Target, which allows the player to select a space for the unit(s) to move to, Roam, which lets the allied unit(s) move wherever they can, and Halt, which prevents them from moving at all.
In Path of Radiance, Tanith is capable of summoning allied units. She can summon up to six ally units (4 Pegasus Knights and 2 Falcon Knights) to help you during battles, which can come in handy during situations where you are surrounded by enemies.
In some Fire Emblem series (Fire Emblem: The Sacred Stones; Rekka no Ken; Fire Emblem: Binding Blade) Neutral Units act as Ally units since they fight alongside with the Player Units against Enemy Units.
Ally units, like any non-Player Unit, receive no experience for combat. In Path of Radiance, allowing ally units to fight for you will cost you the experience one of your player units would have received otherwise. In Radiant Dawn, however, experience points that ally units amass will be received as bonus experience at the end of the map. Additionally, keeping a certain number of ally units alive in a chapter often results in additional bonus experience.