This isn't a general assassin-type story where the killer kills in cold blood, no sentiments involved, all because he tries to avenge the death of a certain loved one. No, in fact, the story gives us a side to how far circumstance can drive, us, as humans, to respond.
There are quite a few number of characters that get a decent exposure in better understanding their own circumstances of life (largely because they play a role in molding the character that is Kai), outside of the main character's own development.
The art in this one is fantastic. The author really pays attention to detail, which,
in turn, gives the reader more reason to be captivated by the story.
Kai's story comes to life in the present. There are flashbacks, but they are there to allow the reader to understand the kind of life Kai led before becoming an assassin and not the reasoning for which he acts like how he acts (as the sentimental assassin). That way, the events that bring Kai's story to life, allows the reader to understand that a person's actions should be judged by the present (and only seen through the past). The reader is given a kind of story where a person is making decisions by himself, in judgement, through close contact with other environmental/social issues. (Which is one of the bigger themes.)
There are a few scenes where the killer, Kai, gets involved with other men in an intimate fashion, but not something that should baffle someone so much as to want to quit reading it. Butttt, if you live for that boys love intimacy than you will especially enjoy a specific scene in the series. ( ͡° ͜ʖ ͡°)
OK. But really I'll shut up now. Haha.
Overall, it is a really great read. Anyone wanting to pick up a story that involves a very attractive cast of characters (both how they develop and are portrayed physically), alongside a nice plot development (considering how no one event was randomly thrown in to satisfy the development of Kai) , than pick this one up, up, up!!!