Instruments are special 'weapons' introduced in Valkyria Chronicles 2 that either buff allied units or de-buff enemy units in a wide area.
Instruments In Valkyria Chronicles 2
Instruments are carried by 3 sub-classes that branch from the engineer tree. Anthem Corps and Anthem Elites play instruments that buff allied units while Melodists play instruments that de-buff enemy units. While anthem corp used instruments will buff only a few key stats, melodists/anthem elites instruments will de-buff/buff nearly all stats other than health.