Adjutant is a game mechanic in Fire Emblem: Three Houses
Once Byleth has reached Professor Level C, they can select a character during a battle to serve as an Adjutant for a deployed character. An Adjutant character participates in battle as a supportive unit, assisting in their Battalion and providing stats, but otherwise does not directly participate in battle. In addition, there are three sub-types of Adjutants, based on the character's class. This includes Offensive (indicated with a red sword), Defensive (blue shield), and Support (green sparkles). Offensive Adjutants increase the main unit's ability to perform Follow-up attacks (needs confirmation), Defensive ones may negate enemy Follow-up attacks, and Support characters will restore health to the main unit at the start of each turn.
The Adjutant does not affect the total deployed characters and are considered a separate entity in battle. Depending on Byleth's Professor Level, one to three characters can be chosen to be the an Adjutant for a character, however only one Adjutant can be assigned to a deployed unit.
Adjutants passively gain experience, pertinent subject proficiency points of the weapon they have currently equipped, support points with the lead unit if applicable, and class mastery points whenever their leading character goes into battle. For offensive Adjutants, even if they perform an Adjutant Follow-Up attack, they will not consume a usage of their weapon.
Furthermore, if the main unit in an Adjutant pair is a flying unit, the Adjutant will be required to be a flying unit as well, however; flying units can be assigned as Adjutants to any unit, regardless of the movement type.
An Adjutant is a military officer who assists a commanding officer with unit administrative duties.