I found the initial premise for Happiness to be intriguing. It starts out exploring how the humans-turned-vampires cope with their new lives. How they handle this new state (e.g., the thirst for blood), how it affects their relationships with others, and the struggle they face to remain “normal”. I was pleasantly surprised to find an absence of overt revenge or power fantasy elements. While this type of story isn’t anything novel, I liked the dark, gritty take on the “underground” lives of vampires.
Approximately halfway through the story takes a turn for the worse. It starts to branch out into different arcs, some of which take the story in what I thought were very ill-thought-out directions. The first half has what I consider to be a much better composition (i.e., it was put together better; more compelling and coherent) than the latter, which was just generally haphazard and shallow. Even ignoring the questionable direction, I don’t think there was enough time given the length of the series to develop these new arcs adequately. While I genuinely enjoyed the beginning of this story, it had almost completely lost me by the end.
The main cast is interesting and I liked the different ways in which they are initially developed and brought together. As mentioned by the mangaka (in his note at the end of volume 10), he tried to explore the psychology of multiple characters (while maintaining character quality) as opposed to focusing on just one. In this respect, I think he was mostly successful and I generally enjoyed the different character narratives and perspectives.
One of the areas of weakness for Happiness which really detracted from my enjoyment was the failure to development some of the more significant supporting characters. The only one who really gets proper attention is Nao. Saku appears at multiple points over the course of the story, including some important events. He has significant impact on the outcome of some characters' lives, but he doesn’t even get a short backstory (or anything else, really). Nora was the biggest failure, I think. She is the catalyst for Makoto’s vampiric transformation, a guide for his new state of existence, and his love interest. Despite this, her backstory is introduced shockingly late in the story, her motivations are barely explained, and no romantic development is allocated to her relationship with Makoto.
I really like the art and think the quality is objectively good. Things like the way the mangaka drew the sky and other perceptive differences for the vampires added an interesting character to the visuals. This is the one element of quality that remains consistent throughout the series.