When two units like each other very very much, they gain stat boosts in battle. Supports have ranks, and the higher the rank, the stronger the boost.
Mechanically, S rank and A+ supports are the same. Units may not have more than one S/A+ support. S rank denotes a romantic relationship while A+ rank denotes a platonic relationship.
Support pairs may be added/removed as more work is done with the game. Ranks in gray denote no scripted conversation for that rank yet. Pairs listed have at least one conversation in their chain fully scripted.
Please note there are unmarked spoilers for characters' backstories in these support conversations. Read at your own risk!