If I wanted to explain all the reasons I loved this, I would spoil everything; thus, I'll have to speak very abstractly. Nevertheless, I hope my enthusiasm reaches you.
Story: 10/10
Absolutely fantastic. The mystery and intrigue blended well with each character's actions and story, leading us along an insane locked room detective adventure, complete with battles, theories, facts, and a grand climax.
Art: 9/10
The characters had great designs and the locale was very vibrant. Nothing struck me as incredible, and at times, you could tell the production value wasn't massive. But it still gave a positive impression. Great color scheme as well.
Sound: 10/10
Some of the tracks were just perfect. One of the battle themes, in fact, is stuck in my head even now. And the OP music style changed to match the mood, which was a great touch.
Character: 10/10
The character development is great. Being a mystery, the rapid changes, theories, backstories and facts all mix together, leading to a race of thought: well, this guy has this backstory, which might suggest this; but this other chap is unknown in this regard, so maybe he might be the one... And so on. The intrigue only serves to drive character development forward rapidly in an excellent manner.
Enjoyment: 10/10
I've always loved mysteries blended with action. This fits the bill. Everything worked out wonderfully.
Overall: 10/10
I am very glad to have watched this.
