Got a few typos from the prologue:
And as for you, the third member of the group…
After Emilia, Ren, and Silvio, shouldn’t that be “fourth”?
Now, there was actually a fourth group member missing – Cecilia.
Again, should be “fifth”.
She was supposed to sleepover like the rest of you yesterday, but ended up leaving a couple minutes before midnight, claiming her mother needed her home.
I’m not certain, but I think “sleepover” is a noun (as in “we had a sleepover”), while “sleep over” is the verb (which would go here).
It was Saturday morning, and you had plans about half an hour ago with (speak of the devil…) Cecilia.
“Speak of the Devil” is generally used idiomatically to denote the arrival of the person you’re talking about, not just a further mention of them, so it doesn’t really work here.
Even though she had plenty of friends around town (or “allies”, as she sometimes called them. You could never tell if she were joking), she seemed to prefer your company, along with Emilia, Ren and Silvio.
I’m not entirely sure of the grammatical rules for sentence breaks within parenthetical statements, but I’d avoid them, maybe with the following: “…or “allies”, as she sometimes called them; you could never tell if she were joking…” Also “if she were” and “if she was” have slightly different meanings, and I think “if she was” works better here.
Sorry there are so many. Good luck with the game! 