In the final chapter of Volume 9 of the Yuru Yuri manga, Sakurako was feeling sleepy in the student council room. At that time Himawari stepped in, apologizing for being late. But Sakurako, who was in dreaming mind, saw Himawari's face as Akari's and shouted out 「あかりちゃんの裏切り者！！」. As I understand it, she is either calling Akari a betrayer, or saying "Akari's betrayer", but neither of these interpretations makes any sense to me. What is the real meaning of these words? Why did Sakurako say that? I know this manga is not meant to be difficult to understand but I just cannot get my head around it. So please explain that to me if you know the answer. I would really appreciate your help!!
p.s. This is the only one place in this chapter where Akari is mentioned. So I guess the answer may lie in previous chapters? But I do not have access to all volumes from 1 up until 9.