Man, finding a straight answer on 'Ano Ko no Kawari' is a nightmare sometimes. Everyone focuses on the online serials. Let's break it down.
It's mostly about Oono Sui, this shy, bullied girl, and Yuuji, the guy she hires to pretend to be her for a while after she gets fed up. The core dynamic is between those two, with her watching from a distance as he impersonates her. That's the whole engine of the story.
Then you've got the people around them reacting to 'Sui' acting totally different—like her sister, her classmates. Yuuji's friends pop in sometimes, usually to call him out on his weird new gig. Honestly, the cast isn't huge; it's a tight, character-driven thing. The author really digs into those two alternating viewpoints, especially Yuuji's struggle to understand Sui while pretending to be her.
I got halfway through the web novel version before losing track. The manga adaptation might add some side characters for flavor, but the heart is that strange, transactional relationship between the leads.