@SasazukaIchimashi Back when Haruka's friends, mainly Maki, were wondering how to act like a delicate girl to attract male attention, Maki tried to make an example of being unable to open a soda can and asking for [a guy's] help for it. However, her acting didn't seem too convincing, so she suddenly asked Atsuko to open a can. However, Atsuko genuinely failed to open one and asked for help in a cutesy manner, impressing Maki. Or something like that.
That's pretty much what I remember of Atsuko. She's the shy, gentle girl. Maki is more energetic and social, and is approached by Hosaka occasionally much to her annoyance (disgust). Hayami is of course the alcohol (exotic juice) smuggling girl, who's messing with Hosaka all the time, laughing at his inability to approach Haruka. Btw, the characters aren't all classmates since they are from different years. Hayami and Hosaka are third year. The Haruka, Maki, Atsuko trio is second year. Natsuki and Hitomi (the girl who loves Natsuki and has a delusion she can read Natsuki's mind) are first year.