" Your drool is really, really sweet, Hayakawa ! What does that mean . . . . ? "
Seo is only shown once in the anime, and that one time he is only shown from his side and from the neck down. In the manga, Seo is also shown only briefly, when he attends the Hoshinome School Cultural Festival. Seo attends when a friend of his calls him and revels that he has just seen Hayakawa walking arm-in-arm together with another boy, as boyfriend and girlfriend.
Originally, Hayakawa Aika was interested in shipping with Seo, but for that period of time in middle school, nothing mutual ever developed, due to the fact that Hayakawa's love towards Seo was an unrequited love. This because Seo, at that same time, was himself interested in another girl, but Seo's love for her also was an unrequited love, since that girl was not interested in Seo.
About the time that Seo began to have a romantic interest in Hayakawa, and wanted to help Hayakawa rekindle her love for him, Hayakawa was interested in trying to get Tsubaki to rekindle his love for her. Thus, as Seo repeatedly made overtures to Hayakawa of his love interest in her, Hayakawa was not listening, or if she was, was ignoring him. This lead to an argument between the two of them, with Hayakawa telling everyone that she had broken off any shipping with Seo.
Finally, on the day of the Hoshinome School Cultural Festival, Hayakawa finally was made to understand that she will not be successful in forming a drool bond with Tsubaki, and rekindling his romantic feelings for her. But at that time, later in the day at the cultural festival, Seo makes an appearance, and for the seemingly hundredth time, told Hayakawa that he is willing to have her rekindle and restart her love, as he is also redirecting his love towards her. On a whim, Hayakawa offers her drool to Seo, with a most erotic and violent reaction on his part. Their love for one another was proved by the offer and reaction to Hayakawa's drool, and the forming of the one thing that was most precious to Hayakawa. That being a drool bond based on a true mutual romantic love and an erotic shipping, a drool bond with the boy destined to be her life mate and lover, the boy who she would be destined to have sex with for the first time !