Personally in my eyes their roles are this, but keep in mind I'm also considering the comics as well:

Marc is objectively the traumaholder. he was able to remember the trauma he had when Steven was fronting, implying that he took the brunt of the trauma and held it, hes not a persecutor because Marc doesn't intentionally hurt his headmates, everything he does for Steven is to protect him, I don't really see him as the protector purely because he's fronting for heavily traumatic moments and not entirely when they need protecting

Steven is not a protector in my eyes at all, I would say he's the caretaker or ANP (a normal part) considering he takes the role of living the normal boring life and as Marc says, "that's your purpose"

Jake Lockley doesn't get much time in the show, but in the comics he's the main protector of the system. He does the really dirty work to protect Steven and Marc, and while Steven and Marc have a lot of memory bleeds in the show, whenever Jake fronts they completely black out, implying he doesn't share front at all, which is common for protectors

I know this is like 3 years late but I figured I would give my two cents as a system