Some general spoilers ahead, synopsis of first chapter.
If you don't mind a lot of senseless callous violence then this can be an exciting romp of an adventure series with lots of cliffhangers to tease you and pretty men to smoulder at you. I read all four volumes because the story is good at suspense and I was interested to find out what happened next but I never could relate to or sympathise with any of the characters so never cared about any of it.
The first chapter is terrible and the rest of the story just never quite recovers. Poor Florian is sold to Ray and the first thing Ray does is to indulge in a spot of torture. Don't imagine some sexy sort of teasing torture, nope, it's just straight-up torture with whips. Halfway through he gets bored and lets his henchmen finish off the job till Florian loses consciousness. The henchmen and Ray's loyal sidekick look on in chibi form with mild worry. Haha, are we supposed to find it funny that Ray is a callous sadist?
Ray is impressed that Florian suffered through it all without complaining and falls for him, I guess. Later on in the chapter Ray saves Florian from something or other and Florian falls for Ray, conveniently forgetting all about the torture episode. The whole setup is frankly ridiculous.
I pity poor Florian throughout the story because he goes through even worse things. I don't feel an iota of anything for Ray because, unlike Florian, I can't conveniently forget the little torture incident.
Later on we're introduced to a beautiful baddie with an eyepatch and long flowing hair. I think we're supposed to find him hot but unfortunately he has a habit of engaging in cold blooded murder and yep, you guessed it, torture; I don't know about other girls but that's rather a big turn-off for me. He, like Ray, has a traumatic past that explains away the behaviour but to me it's all somewhat contrived and trite.
Some other reviewer mentioned depth but I have to disagree. The characters are more like caricatures. Ray swings wildly from randomly evil to randomly sweet. I think he's supposed to be the troubled antihero sort but he lost me from the first chapter so I don't really give a toss. Florian is a sweet noble guy but his acceptance of Ray is wholly unbelievable. He's the typical innocent martyr type whose goodness verges on saintliness. The eyepatch baddie is just evil all the way, I don't get the attraction - is it the eyepatch?
The story actually has some potential, Higuri spins an interesting tale of adventure and a possibly intriguing relationship between Ray and Florian but unfortunately the whole thing just falls short.
I rated it a 5 because it kept me reading till the end and I was willing the characters to develop, well, more character, but they sadly disappointed.