(SPOILERS)
rule of thumb: if the protagonist of an rpg maker game is a child, they probably have killed/will kill someone.
i think that, out of all your games, this is by far the scariest one. the combination of the atmosphere, the mystery of the story, the haunting music, the creepy artstyle that elevated every event... it was all incredibly eerie. at some points i physically recoiled in my seat cause i thought a jumpscare was about to happen haha. at the very beginning, when you can't move and suddenly two children slowly disappear, that set the tone up PERFECTLY. man this was so good.
the story was really intriguing!! the idea of a children "slipping off" out of the normal world, and being watched over this evil spirit that wants to play with them, is really really creepy. and the fact that this kid is willing to go to another country just to find his little brother, because him disappearing made his parents not even care about him... it's really tragic, but really compelling. even if this isn't a GANA game, i really got the GANA vibe of "something supernatural happening that only a few people can see"! i feel like the beginning is a bit confusing, since it's not immediately apparent if it's connected to charlie or not, and i kept thinking "wait am i playing as the brother of the two kids who disappeared?" but that didn't prevent me from getting hooked into the story so it's not a big deal. oh, and i loved the unexpected twists and turns the plot took! charlie killing the monk was SUCH a shock haha, but it didn't feel unearned, and i think part of that is because of just how much information is always kept from you. every step of the journey is filled with so much mystery, from the opening scene, to meeting the monk and not knowing if he's lying, not knowing who these spirits are or what do they want (the moment they appeared, i ran away like a coward until charlie said they weren't hurting him lol, such a cool moment :D), finding strange places like the wolf head altar, exactly what even is rirou... i think i would've preferred to have a bit more stuff explained, but it's undeniable how much intrigue and dread comes out of the mystery of the plot. really great stuff.
speaking of intrigue and dread, i REALLY love this "other world" where the rirou lives. everything about it is just... incredibly eerie. the all-consuming blue hue, the seemingly random traces of the real world (the trees, fences, streetlights...), and of course the "ghosts" and their strange games. and sequences like the music game and tony made me the most tense that i've been in a while haha. and like i said, not knowing anything about this spirit or this world really helps in building that atmosphere. you said in a comment that you based this game on a dream you had, and honestly it's easy to see it, it has such a dreamlike and ethereal feel to it.
in terms of presentation, wow! this game's visuals are so unique, and great with conveying the feel of everything. the ghosts are super creepy and i love how blurry and ethereal they look, it's hard to make a distinct ghost design but they were extremely memorable! i love the narration text at the beggining and the end, the opening sliding doors at the start of each part, and in general the use of japanese architecture and symbology. really dig how handdrawn the original art feels, damn this is such a beautiful game. and the MUSIC!!! oh my god, the music was amazing. every single one of the tracks was absolutely incredible, and captured the feeling of each sequence perfectly. it feels like it creeps into your head and doesn't let go, forcing you to experience this twisted story and feel as disturbed as possible. it elevated the unease i felt sooo much. the presentation was just fantastic all around.
like i said, i feel like some more things could've been explained, like what is the altar, what do these games the creatures are playing mean... it's not too bad (and in some aspects it does support the "dream-like" feel of the game), but part of me wanted to get slightly more answers. and there's this one point right at the end, before charlie and henry see the monk, where charlie says "i see it all now" and some images of the ghosts appear... honestly i didn't really understand what happened, and i don't know to which extent that's my own fault lol.
overall this was great!! a twisted, bittersweet nightmare that gripped me and didn't let go for a second. if i played this as a kid i would've probably fully believed "slipping off" was a real thing and subsequently would've been scared shitless XD