Secret Chars:
Undyne Undyng: Kill Undyne
Mettaton Neo: Kill Mettaton EX
Folower 1: talk to the river man
Folower 2: interact with the artefact until a message appears
Folower 3: Interact with the phone until entry 17 sounds
Goner Kid: Search Goner in the searcher
Chara: Kill all the monsters except MysteryMan
[REDACTED]: Kill the 3 folowers + goner kid
So Sorry: join to the game 10th October
Mystery Man: can appears entry 17 in the menu it can appears with a 0.34% of luck i got it today