Here's a small explanation of how I went!
-Sergio: His outfit is pretty much an "historical" one - it is basically his Mai Dire Fine outfit, but worn with the jacket open instead of closed (something he did with his black PPC one). I felt it was what worked best for the art style.
-Nikki: I went for a replica of her Barrier Jacket, and I was very lucky that in the limited selection of outfit pieces I managed to find what I needed for a good lookalike. In fact, I dare say it is more accurate than the one I made in Koikatu. I was also happy that the "hair intakes" bangs were already in, I had to use mods in Koikatu to get that part right!
-Corolla: Her outfit is quite simple, and while the hairdo is different from the Koikatu one, it is still a very Corolla one and she is actually the kind of person who changes it on a whim.
-Ami: Like with Nikki, I went for a replica of her Barrier Jacket. As I couldn't really make it look like the Koikatu one, I just went for getting the "feel" of the outfit right. And, obviously, her one-side-up pigtail is represented as well.
-Kathleen: I was frankly surprised I could perfectly replicate the outfit of her first appearance. The pink-died part of her hair is a bit too big, but that's what Gacha Studio allows.
-Keiko: Again, i went for her Magical Girl outfit to give her a distinctive appearance. This also turned out to be a perfect replica, much to my surprise. The twintails are a bit too long, but it fits the artstyle so I don't really mind.
-Hajima: She unfortunately drew the short stick, as I couldn't get her hairdo to look right. On top of that, the only front hair part featuring the headband doesn't allow to color it. For the clothing, I went for her PPC outfit, but with a magenta shirt underneat to give some color. It is, in fact, her favourite color.