Hey hey! Thanks for checking out the game :3
For the sprites, it was a creator software I purchased years ago from a Japanese site I stumbled across somehow. I don't remember what the site was with it being so long ago, but I remember it saying that the people who made the software were called T.O.P (or something like that) cos they were the only English characters on the page, so it stood out.
I think it was a very popular product! I imagine that it's been used to make sprites for many games + other kinds of projects, so you probably would've seen the art style before :3 The great thing about the software though is that there are so many individual pieces to create hairstyles from, lots of eyes, lots of clothing and accessories, etc. that you can make some pretty unique sprites with it :D It's just a shame that they were mainly for making female sprites and not much available for male ones :(
It wasn't just a one off purchase though. I think I had to buy it in parts. Like each part has 1 base body and a whole bunch of stuff exclusive to that set. It wasn't particularly cheap, cos I think I paid about £150 for 10 sets to have enough base bodies for the number of sprites I needed. Still, I guess that's much cheaper than paying an artist to draw the sheer number of sprites I used, haha.
Since I can't actually read Japanese, it was a bit of trial and error to use it x3 The interface is simple though so you can just kinda figure out what does what by changing things and seeing what happens. It can be quite laggy sometimes >.< so it took a looooong time to make all my sprites, but I got there in the end xD
Also, I think they might have been designed for use with 18+ stuff cos you can take their clothes off, have em in some suspiciously suggestive naked poses, and make them pull ahegao faces, haha.
Sorry I can't be of more help >.< My memory kinda sucks x3 If it was all in English, I might've stood a better chance at recalling more info, but with everything being in Japanese, that's about as much as I can remember about it!
I hope you enjoy the rest of the game :3