Huh, looks like chapter 12 was Zumi's big infodump chapter. I wonder if she was even named earlier. (I think no.) She spoke up in 7, surprised that Umi was getting along with Shou, and telling her about the "eyes".
Looking at 13.2, a black ponytail with camera talks to Shou during the culture festival. Seems somewhat friendly, and looks a lot like Komatsu from 17.1, but she's called Kosugi. Then joined by two girls who look a lot like the rest of Shou's group. But 'brown' calls ponytail 'Kosugi' in 13.2, 'Komatsu' in 17.1. Unless they've changed intimacy since then, suggests it's really two different yet similar groups. Pity, I would have liked more continuity.
Kosugi-chan (to Umi) again in 14.1, with the "you're like Segawa's mom" line.
Also in 13.2, Ken is running a candle stand or something, and Ibu is... sitting to the side, looking away angrily. Quel surprise. I dunno what she's mad about, other than that being her default state when Umi isn't paying attention to her.
Then the bully group, who look like Umi, a more stylish, Yosshi, and the lovechild of Nae and Zumi. I have to say, clear visual distinction is not a strength of this manga.
I wouldn't bet heavily on Nae being WLW. Maybe she is just more touchy-feely. But she really is (by comparison with everyone else), with Umi specifically, and then her rather aggressive "I'd like to get to know you better" scene with Shou.
I dunno about Baba; she's drawn less conventionally attractive than others, could easily have liked boys without it going anywhere.