I think I get the characters now. Momiji is a bored social climber. Since Roka is the "most handsome" boy in school, she decided that she'd go out with him to make her famous within the school. She constantly makes social media posts for more fame. Roka doesn't really love Momiji because he already saw through her a long time ago but didn't want to destroy their relationship and Momiji trying to isolate him. Miko loves his mountain. Mountains before hoes.
Since it's the boy's abyss author, Momiji will do something scandalous in the future for more fame and most probably in that mountain if someone famous comes to town. It's probably the masked guy who'll see it and will hesitate telling Miko first but eventually do.