Some people seem really confused by this, and I'm not sure why. We know who this is. It's the missing party member. It was pretty obvious from the start, but the last page outright confirmed it when he/she gave their name. This is the all offense zero defense blasting wizard that wasn't present when the party decided to kick the MC out for his own safety. You know, the one where they were like, man, it sure is a good thing X wasn't here, and there will be hell to pay for it when X gets back, they'll lose their marbles when they find out MC is gone.
This is that person. Some people are like, why are you showing this person. Because they're relevant to the plot. The blasting wizard that's going to lose their shit over MC being booted off, is about to come home and lose their shit because the MC was booted off. Maybe MC is "master," I would assume so, since the MC has been training them and XP boosting them to get them to where they are today. It could be somebody else, but it probably is not.
I don't think this is very complicated.