" I took a picture of you drooling at your desk, Youko ! "---Arima
Youko recalls that Arima was her boyfriend in high school. He even took a picture of her at her desk, drooling. When Youko was shown that picture by Arima, she wanted it discarded, but Arima refused, stating that she was so, so cute, with her hair in twin tails, and drooling. After high school ended, he attended a university in the Kansai region. He and Youko stayed in touch for half a year, but slowly grew apart. Because of not staying with Youko Tsubaki, that implies that Youko was never able to Drool Bond with him, possibility because, like Ueno and Ogata, he does not have drool capabilities.
Years later ( as shown in Chapter 85 of the manga ), when they are both young adults, Youko has a chance meeting with Arima. They spend some time together, but the encounter leaves Youko somewhat emotionally upset, possibly with regret that she did not develop a relationship with him. Youko is so moved by her emotions, that she invites Urabe to share a bath together with her in a public bathhouse, and tells Urabe everything that is on her mind about Arima. In return, during the very intimate interlude between the two young women, Urabe reveals that she is an intimate girlfriend of Youko's younger brother, namely Tsubaki Akria.