Affinity refers to how well units get along with each other.
When units with high Affinity for each other are located close together on the map, they will receive a boost to their stats.
As Affinity levels increase, the range and effect of the stat boosts also increases.
You can view a unit's Affinity with other units in its stats details.
*If a unit with stat-boosting Affinity is KO'd in battle, its Affinity will drop.