Which Fire Emblem game introduced the 'Pair Up' mechanic?

Fire Emblem: Awakening introduced the Pair Up mechanic. It lets two allied characters occupy the same battlefield space and act together as a combined unit.

The leading character controls movement and attacks, while the supporting character contributes stat bonuses and can sometimes assist with an extra attack or block an incoming blow. Support relationships grow as characters fight together, making Pair Up both a tactical system and an extension of Awakening’s relationship mechanics.

Pair Up is sometimes confused with earlier Fire Emblem features such as Rescue, which allowed one unit to carry another, or with the series’ long-running support conversations. Those systems influenced the idea of cooperation, but Awakening was the first game to formalize two characters sharing one tile in this way. Fire Emblem Fates later revised the system by distinguishing more clearly between units standing adjacent and units joined in a pair.