"White Parasite" is the culmination of two separate stories that were thought up at different times.
Originally, the title series was much more parodic but lacking the humour that comes with parodies. It would have been magical boys who wore dresses after transforming. Each magical boy would have had a mascot animal, who would turn out to be a human who had been forcefully converted into a mascot form.
The second story, "Step Up", was explained in the first half of bonus chapter 1, with the exception of Shousuke being called "Saranosuke" for some reason that made sense at the time of conception. Saranosuke was a lizard/human hybrid, but because I needed a reptile that would be less conspicuous, he became a tortoise.
These two different ideas are why Christie and Hazuki are, for the most part, out of focus despite being set up as the main protagonists. They don't require much characterisation because they are the most willing to take part in the magician system.
As for why "White Parasite" lacks a true direction or bad guy, I mostly wrote up chapters as I was going along, spurred on by a rough idea of what would happen. If there were any ideas that sounded good in my head, I threw them in, so the ending is a lot more different than it was envisioned to be. It's a lot more tragic too because originally, Hazuki would've destroyed the dimension machine while Christie held Luna off, but somehow, the more tragic ending gave more of a moral so it became that ending. (Also, after "One Wish They Never Wanted", I've found myself incapable of writing a proper antagonist ever again.)
This is the story of many, so in the end, "White Parasite" became a character study for why people do what they do and how the strength of their relationships can make people do things against their own will.
United we stand, divided we fall.
That is why in both a literal and metaphorical sense, Hazuki is the eponymous White Parasite.