The main story was really bad. Aion makes this extremely convoluted and unnecessary complicated plan to do something, and just ends up dying anticlimactically at the end. After watching the entire series, i honestly can't tell you what Aion was trying to do, or why he was doing it.
The last 12 episodes are so bad it brings this down to a 3. Characters do things for no reason. Nothing is explained. By the end, i didn't care the slightest bit about any of the characters. I didn't care it they lived or died, or what happened to them. The last episode had more characters doing inexplicable and random things that make more sense, but there is one scene where Rose and Chrono die. It's actually the only decent scene that makes sense in the last half of the show. It's actually a great scene, one might say beautiful, but I didn't care about the characters so i felt absolutely no emotion in that scene. What a waste. I felt this scene was good enough to raise my rating to a 4.
I didn't really like the art. It wasn't bad, it just didn't fit the series. It seems too cartoony and happy for something trying(and failing) to be deep and dark. Chrono in his true form looks goofy and stupid.
It should also be noted that the author of this has no understanding of christianity. I don't understand why animes keep doing this. If you're going to butcher chistianity, just make up a fake religion instead of trying to be clever.
