Beneath the Mask is a song from Persona 5, and the flavortext confirms
this connection by mentioning a "group of thieves". The flavortext
further describes fusion within the Persona series, so this must be
relevant. Furthermore, it is said that there's nothing regal, so
we do not use Persona 5 Royal, but rather the original game.
It turns out that all four feeders in this puzzle are the names of Personas. Also, because there are six blanks, this clues to take every pair of Personas and fuse them together. There are many fusion calculators online that compute this for you, such as this one.
We receive six Personas: