I don't want Mephisto to be "the big bad". He's not good but he's not really evil either. It'll be too cliche if he ends up being the final boss.
Despite I don't like Mephisto to be the big bad either, I also don't enjoy too much if Mephisto becomes the one to save the day too often (happened in chapter 116 & 118) . I wonder whether Kato will continue keeping his ambiguity ...