I actually agree that Agawa is pretty underdeveloped right now as a character. Most of what we know about her comes from other people talking about her or from how she acts, not from her own focused arc.
I am not saying “actually she is super developed,” because I do not think that is true yet. I am saying the setup leaves a lot of room for a later turn where the focus finally shifts to her side, her head, and her healing. If the manga ends or moves on without ever giving her that, then I will fully agree she was wasted. Right now I am just holding out hope that the current imbalance is intentional build up for a future Agawa centered payoff
The thing is that Agawa basically already had her arc at the very beginning of the story, when she was the primary focus of the first just under 3 dozen chapters before Akito stole the spotlight for her own arc. There's really not much else the author can really do with her.
And I think that the main problem with Agawa is that while she is an eye catching, good looking character for the cover, and a fairly true to life depiction of a high school aged slut with a heart of gold (save for the exaggerated body count); that doesn't mean she's an interesting/compelling character at her core. At least, not an interesting/compelling enough character to sustain a long running manga on her own.
Think of it this way, who is Agawa? A slut. Why is Agawa a slut? Because her (and presumably Shiina's) sleezebag father knocked up her skank mother, she was raised by an aunt that doesn't really seem to care what she does, and because her perhaps more mentally fucked up half-sister pathologically seduces every boy she ever liked. What is Agawa's purpose in the story? To reform her slut ways via Gotou's Godtier innate sexual prowess and single-minded devotion to her. What else is there to know about Agawa? She likes baseball, she's generally nice to everyone, does fairly okay academically maybe, and I guess she lezzes out from time to time.
This was enough characterization to go on for the original little h-manga, but do you really think anyone would read 100 chapters of just Agawa fucking a thousand different men before forming a contrived exclusive sexual/romantic relationship with Gotou? Answer is no, and hence why she's been relegated to a walking plot device in Gotou's story, sidelined in favor of exploring Gotou relationships with far more appealing (IDK about anyone else, but all those detailed panels of Agawa fucking dozens of men that aren't Gotou, just plain grosses me out more than a bit regardless of how attractive Agawa is) and interesting heroines.
Guaranteed that the ending will just be a rehash of the h-manga's ending where it doesn't really matter what they did in high school, because they meet again around a decade after they both got expelled and go "eh, why the hell not, let's settle down".