While I think that Marcus might own a OP Isekai Protagonist label, nothing about him screams any of that. In fact, with what little we have of Marcus does not even claims that he is from our world (or one that is adjacent) or from the world the VN takes place in. The OP Isekai Protagonist label is on him is due to how quickly he learned to use magic with Silas saying "[Marcus] managed to it on [his] first try."

While I don't know how it fits, but I think a greater power has plans for Marcus with the area for his Major was blank for a moment prior to his PAL lighting up like a flood light. Even the staff member said that it was blank prior to him receiving the PAL. In addition to him not even existing.