Broooo you just don't understand broooo
He's so kind and nothing else brooooo
I don't really care the girls are cute and fun enough to compensate lmao
I understand why Amane came to like him, they started connecting thanks to their shared love for Kiramon. And even if girls flock to her, we saw when she met Ijichi she has zero social inclinations so that's another thing in common for them.
But Ijichi getting interested by him and falling was random, no matter if saying so rain's on the parade indeed. The way she explained it, it seemed less about Seo truly but more about timing, she was interested in romance and she happened to get along more and more with this guy.
I love reading this but it's good to remember that this never happens lol. In reality, Ijichi would have dated any other of the much more confident prospects available to her without wanting to wait for some nerd to realize he's worthy of love.
And same for Amane, she'd have kept Seo as her Kiramon friend. Girls are very good at developping genuine and fulfilling relationships with different people to create a wide support network. That doesn't mean that the dude being invited to join this circle is automatically jumping to the boyfriend seat.
There's no urgency to not keep it for a guy that is exactly her type like that idol looking like her Vamp-sama.
Having said all that, the one thing that made all those feelings blooming a little more believable is that aside from their group of three, they're not close to anyone else in school. Amand and even Ijichi only have superficial talks with them, their school friends don't know anything about their family, true interests or personality. Seo does.