As the Master at Chaldea, you act as the player character. You'll voice the multiple-choice menu selections.

As a self-insert for the player, The Protagonist has absolutely no background details or connections to other people or events. They are simply "someone who just happened to see a recruitment flyer at a station, applied and got accepted into the job, and somehow ended up getting taken into the Chaldea Security Organization", effectively having joined Chaldea on a whim while being a civilian with no training or combat experience. They are acting as a newly established Master candidate of Chaldea whose only purpose is shoring up numbers.

It is repeatedly mentioned that they are below drinking age.

Open to any interpretation voice-wise - the gender of the Master will be determined by the range of the voice actor selected.