Adjutant

Adjutant is a game mechanic in Fire Emblem: Three Houses

Overview
Once Byleth has reached a certain Professor Level, they can select a character during a battle to serve as an Adjutant for a character. An Adjutant character participates in battle as a supportive unit, assisting in their Battalion, but otherwise does not directly participate in battle. 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, support points, and class mastery points whenever their leading character goes into battle.

Etymology
An Adjutant is a military officer who assists a commanding officer with unit administrative duties.