All characters in KOF vs. DNF are unlocked via the main story mode (Arcade Mode). There are no microtransactions required to unlock the base roster.
If you want all characters immediately, enter the following code at the main menu: kof vs dnf all characters unlock
Hold L1 + R1 (or LB + RB) and press Up, Down, Left, Right, X, A, B, Y All characters in KOF vs
Or input the cheat via the options menu:
KOFvsDNF_UnlockAll = TRUE (PC version config file) Hold L1 + R1 (or LB + RB)
In KOF XV, most base roster characters are unlocked from the start (39 characters).
However, some require DLC or special unlocks.