About this Sheville just shown and revealed like Satan, here a short summary of the chapters we saw him before:
Here a short summary of what I understand:
Ch 21: Gwangnam beats the first reaper.
Ch 22: Gwangnam is taken by Sese to Mt Balck Coffin, a place she tries to use to get rid of Kwangnam and to take his ticked (all of this following her mom recomendation). Inside Gwangnam's dream, he meets his old friends.
Ch 23: In the dream, inside the dream, his friends became dark ghosts and take him to meet certain guy that in…
Ch 24: it turns to be Sheville who makes certain contract with Gwangnam, once he chooses Incomplete Beginning option over Complete End option. Thanks to this contract. Gwangnam gets a new form (associated with those skeleton hands sometimes cover his eyes; that also happens to Sese, so likely the contract is something Satan impose on all the gosth who enter the hell).
Ch 24 and 25: Sese can't take Gwangnam's ticked because she's stopped by Gwangnam's new form. She confesses what she was doing to Gwangnam and also she says maybe his new form means he's the Hellper, who seems to be a very rare person who visits the Hell and gets help from the very same Hell.