But after a while Ella notices that everything has a reason, part of a bigger plot.

I have a theory regarding the next characters and their possible abilities.

First, we already know that MP aka Mephistopheles has 7 unfinished games and there is 7 seats, in the fairy tail group they have, there are Pinocchio, Alice, Ariel, Jack. And three missing, is it a coincidence that there are 7 games and 7 seats? I think not. I think that this characters like Jack, Alice, Ariel, and Pinocchio and the rest came from the respected games made by MP. With that in mind, we can assume that they also has the same experiences as Alice, being from a normal human that is addicted to the game, and then transported in the game as characters in their respective games, being boss enemies/supporting characters and such.

Secondly, their abilities. Might also be affected by the game they came from and their characters played in the game, an example of this is Ella having the abilities to manipulate mirrors, and when she became a devil, it has been changed to a mirror that can summon ghost stories and store items and such, the mirror ability came from Ella, and the ghost summoning came from the game she was in. There is also one more proof, Pierrot, Mrs. Spider and Mary are also in the mirror and the junk demon as well. And also one more thing, Alice in the Wonderland and being Alice in the story might've also affected her abilities as well to an unknown degree, but from my observation it might be a safe haven from monsters and ghost stories to yokai's. With this knowledge we can guess and have an hypothesis on what are their abilities.

The next character will might be, Humpty Dumpty, or Big bad wolf, or Little riding hood.

In fact this is just a Theory, a Novel Theory!!

What I like the most though is the fact that the MC tries his best to make the horror game that ends with a bad ending into a happy ending, which is always the reason why I usually avoid the horror genre. I like the concept of Alice who turns 'fear' into 'amusement' and 'horror' into 'adventure' because despite all the danger, they all came back safely, as if they just entered a haunted house in an amusement park.

Like for horror movies for example, even when the protagonist and their pals tries their best to defeat the ghost, more often than not I see the ghost not fully vanquished just biding their time for their next attack, they failed, or it is a pyrrhic victory.

Some people I know often says that it's more realistic that way or whatever but well... the reason I got into reading in the first place is because it's doesn't have to be realistic, because it's fiction. There are already enough suffering in real life, of hard work that ultimately doesn't pay off. I like to see at least fictional characters got their happy ever after.

Well despite what I said, some people do die in the latest chapters but well... it's still a kinder story than most horror genre I've seen.

Not only the transition, the arc itself is confusing like how did the MC get the ability to basically summon literally any monster that is related to mirrors, and then acting like it was the norm, bro you were just a regular guy whose body got stolen and killed off, from where are you pulling all this out of I get that it is somehow related to the MC killing the other 3 half-devils and becoming a complete devil, but the author just brushes off this moment

the MC starts having identity issues due to the personality changes

after the ending of the first part of the story the MC ends up as an SCP-like entity (they also have the classes, but instead of Keter is "Unchained" and so. And the foundation, in this case the "organization" is the best depiction I've read of them.)

in the first part it revolves around some trinkets and special abilities of monsters, which have weaknesses, the protagonist's is that he/she is cooked without mirrors.

However the second part battles does a 180 and it's a full on SCP class annomalies vs other annomalies, the MC also manages to counter a reality erasure by summoning the verse version of the SCP verse Scarlet King.

during the first part the MC as a ghost is influenced by the perception of the children he is with, if they think the MC is ticklish, over time he/she will become extrenely ticklish, etc.