Genesis was a forced addition.
Yes, he is a different character and the character isn't named Gackt, but you can't really argue that the role wasn't made into what it is because of Gackt. This happens in movies all the time. They may write a character over again just because of the big star, even at the cost of the story if it will guarantee them money. And Gackt has quite the fanbase. You can't possibly deny that this was a major influencing factor.
Personally, I found Genesis to be quite flat. You can't label bad exposition/lack of characterization "mystery" and "craziness" and get away with it. It's quite obvious in this case. I ask myself what CC would have lost without Genesis...and find myself saying "not much". SE tried to kind of integrate him into the story, and of course they are going to say they didn't just make the character for Gackt, but actions speak louder than words.
Genesis did not offer much to Crisis Core and was there to guarantee them another door from which money could come in. I think that is obvious now.
His motivations, his dialogue, his backstory (whatever we have), it's all so generic and bland, as if it's plucked from "angsty/obsessive and misunderstood villains 101".
I don't expect Joker in TDK here, but I don't want to excuse this character's poor development because it's a videogame. This is the kind of attitude that perpetuates the idea that video games can't be quality entertainment, or offer some solid characters/stories.
Meh whatever. Angael was pretty generic too, but nowhere near as bad as Genesis. Clearly they weren't made to take center stage, but I don't feel they adequately did what they were supposed to either.
Of course I can rationalize their behavior and see what's going on and why, but that doesn't mean it's not poorly done.