Hold on, you mean 'Aki Sora' the manga? To be specific, it's been a while but the central trio is the twin siblings, Aki and Sora, and their sister Nami. Yeah, it's that kind of story, heavily focused on their... complicated and taboo relationships. The dynamic is almost entirely driven by these three. You've got Aki, who's a bit more on the shy, naive side, and Sora, who is more assertive and carries most of the narrative push in pursuing the romantic entanglements. Nami's role introduces more conflict and adds another layer to the central theme.
Outside of them, there are a few side characters, but they're more like functional props than fully fleshed-out personalities. I vaguely recall a classmate of Aki's named Chika, and maybe a few other school friends who pop in, but honestly, they barely register. The author, Kouji Kumeta, really zeroes in on the intense, claustrophobic world of these three siblings. It's less about a sprawling cast and more about dissecting this single, provocative dynamic from every possible angle, which can make the side characters feel like afterthoughts.
Trying to recall anyone else just makes me realize how little anyone else matters in that narrative. The key characters are, without question, Aki, Sora, and Nami. Everyone else exists just to momentarily shift the tension between them before the story snaps right back to its core. It's a deliberately narrow focus that defines the entire reading experience, for better or worse.