Credits: Pegasus Knight
More detailed information can be found here.
To activate supports, two characters must both participate in a certain numbers of chapters. Once they have fought together for long enough, they will be able to support back at the base. The following table lists the numbers of chapter in the format X/Y/Z.
X = total number of chapters for a C support
Y = total number of chapters for a B support
Z = total number of chapters for an A support
(X – 0), (Y – X) and (Z – Y) are the number of chapters to progress from one level to the next.