@Haen
It's been two years and I'm not the guy you asked, BUT (TL;DR: it's Honoka with Naomi as close second. Also serious shout-out to Mika Mod! Find it; play it; love it.)
I really like Honoka for her infectious enthusiasm, down-to-earthness, and frank sexual desire in what is, at least in part, a porn game.
I also really like how Naomi is a committed Buddhist. Something about it gives her character this depth and otherness that I really get into, even though GTS is not my fetish TBH. Like, usually either a character is a Mary Sue who has astounding self-control because ??? or she has low self-control like the rest of us, but Naomi shows how ordinary, real people like us might go about earning it through practice.
I personally can't get over Alice's meanness and self-centeredness. I admit she has something of a point about ambition - when she criticizes Naomi's class background, both have good points, honestly.
Shiori's notion of order may appear rigid at first, but it's more flexible and wise than that! Like how she wants the MC to not stand up for her honor and reputation when rando students criticize her because: Really, what does it matter? Her character strikes me as simple-but-good; see Akira just below.
Akira doesn't really grab me TBH. Partially because FMG ain't my thing, but mostly because at least in her early scenes she's written without depth. You know, in real life some people are genuinely simple but good: they're loving, hard working, but not clever, unusual, or otherwise sparkly. And those are good people. But they can be boring to read about if the writer doesn't take care to showcase their strengths and what's good about them, and I accuse Akira's writer of this.
Aida... her factor is one of my fetishes, and I actually like the shrinking violet / shy type, even though it's overdone in the anime sphere. The writing in her route ain't bad, either, though it doesn't rise to greatness. One under-developed component is the fact that what she's pregnant with are baby people, and so... what's going to happen with them? Is she ready to be a mother? What about the MC? And so on. I respect the right of a preg fetishist game to deliberately dodge all of that, but Growth Academy is much too grounded and humanistic to get away with it, so it stands out. (It's been a bit since I played her route, so these questions may come up later on, but I would prefer them to show more and sooner, even if subtly in foreshadowing.) Also shout out to making her a big baseball fan! I like that kind of detail that doesn't immediately serve the main appeal, but adds realism and makes for a more well-rounded character.
Finally, I really like the characters and writing in Mika mod. I avoided it for a while because of the mod/fan-work stigma and it was unfinished anyway but DUDE - it's all hot and well-written. I have a couple quibbles but mainly I just want MORE, NOW! It may help that hourglass is my fetish but there's also a chubby girl (who isn't a big fat MEANIE) to appreciate.