You'll need a walkthrough...the human mind doesn't think like this.
All I can say is, it's a good thing I had a walkthrough for this. Some of the "puzzles" in this game are so abstract...who knew the key to that door would be down the hall in that little room with nothing in it on the shelf in the wine glass full of water? >.>
The characters are quite "on the surface." The only one that was remotely in depth was D, the ghost boy. Two endings are possible, based on how well you can help D. Playing through this game the second time without a walkthrough, I got the "bad ending" and found it rather strange. You can get the bad ending just because you didn't look at a certain item, making D remember something with very little importance. But, he didn't remember so it didn't help him. Hm...sad.
In any case, there is very little suspense or tension in this game. For being a mystery, it sure it pretty relaxed.
Bad points aside, it actually uses the features of the DS quite well, using pretty much all of them at one point or another. The plot, although rather predictable, is fairly good. One of the parts, either helpful or annoying, revolves around Ashley's inner thoughts...in the form of a chapter quiz! Oh, woohoo, right? It's not difficult and if you answer something wrong, you'll know. But it can either help you or annoy you.
The graphics are ok, nothing really special. Most of the time, they take on an anime look, which is pretty cool if you like anime (like myself). There is no voice acting, just reading, which is ok. The music however is repetitive and sometimes just annoying/out of place.
Overall, however, if you can find this game, pick it up. It's a good title that got little recognition. Just, as I said, make sure to have a good walkthrough handy...you'll need it.