Once on a winter day, Usaida caught a cold and couldn't go babysitting at the daycare, so he told Kashima over the phone not to worry as there was [Saikawa] to come. For whatever reason, both boys kept in contact without Kashima knowing. At that moment, when Usaida had calmed down a bit because Saikawa was under care, he received a call from Saikawa that he could not understand, but what he did know is that Saikawa had a problem with Midori as he could listen, but unfortunately the connection was not very good, and in the end the call was only cut off leaving Usaida with doubts, who tried to call again, but there was no answer. Although he had a cold, he mustered all his strength to be able to walk and get to where Saikawa was, but halfway there he ran into Kashima and Kamitani, and they both asked him what he was doing standing in the middle of the street. Usaida said that Saikawa had called him about a problem he had with Midori and asked him to take him to Saikawa.
When he arrived at his destination, Usaida saw that everything was in order and then asked Saikawa about Midori's problem, to which he replied that he had had a problem with Midori's back, as there were no dry diapers available. He had called him to ask if he had spares, but the call was cut off before saying that, but luckily, he was able to improvise using cloth diapers, although that was only an option. Usaida felt relieved after that, and then the children had approached Usaida saying not to worry about them since they themselves could take care of Midori, to which after that speech Usaida was about to cry, although it only was a little. Then Saikawa tells Usaida that the others had not been affected by what happened and therefore wished him a speedy recovery, adding that if he could make cloth diapers, it would be fine.
At the Christmas Eve party, Usaida and the rest of the parents can be seen attending the Morinomiya mansion, due to Saikawa calling them to attend the party.
While Kashima was in his studies, Usaida enters the nursery and found Saikawa bowing in front of him and saying that he would be in charge of the daycare, that even if his standards weren't very high, he could take care of the children properly. Usaida, confused, asks Saikawa what was going on, and Saikawa tells him about Kashima's situation, that because he had to concentrate on the exams, Ryuuichi couldn't attend for a while. Usaida then asks what was wrong with Kotaro, because he seemed anxious, Saikawa replies that Kashima and Kotaro had promised to work hard with a pinky promise. When the children felt uncomfortable because Kashima was not with them, Saikawa asks Usaida to pass the baton to him, to which Usaida agrees by high-fiving Saikawa. After reassuring the children, both boys can be showed with the children working on the next scene.
When Ryuuichi was finally able to return to daycare normally, Kashima asks Usaida about Saikawa's favorite food, since they were close friends, he might know about that information.
After Saikawa said a list of his favorite food, Kashima asked Usaida if he knew something about these dishes, which Usaida said that he named all of a variety of french food, but he didn't know about another thing, showing that Usaida could understand Saikawa a little bit by bit.
While Usaida was sleeping, it's shown that he was dreaming about being on an "Alternative Universe" with the kids and the other people. After meeting a grown Taka and a kid version of Hayato, Taka said that he needed take care of Saikawa (which was a baby). Usaida then picked him up wondering if that was really Saikawa, which in response Saikawa says an "Ubuh" as an answer. Saikawa it's shown to be trying to talk to Usaida in his baby form, saying that it was only a dream from Usaida himself, but Yoshihito didn't understand a thing and then he carried Saikawa like an adult would carry a baby with an adding note: "Who would've guessed i'd be holding Saikawa-san like this, instead of Midori". During throughtout the chapter it's shown him carrying baby Saikawa all the time.
In the Everyone's Network special chapter, it's showed Usaida sewing on Holiday, then he gets a message from Saikawa asking if he could send secretly Kotarou's waist sizes by measuring him, which Usaida accepts and Saikawa adds in addition his sizes in the next message. Usaida was a little confused wondering what to do to give an answer in return, to which writes: "I'll make matching PJs for you and Kotarou next time lol", and after sending the message, he receives a happy emoji from Saikawa.
This is a really known ship in the fandom, principally because of Usaida and Saikawa's closeness by drinking in the bars, although is just a part that only the manga readers know. It's the most written for both characters in AO3 and the 4th most written ship in the School Babysitters fandom.
- The "Drinking Buddies" tag comes from the fact that they both tend to go out for drinks sometimes when they are free. Also thanks to this, both boys were able to meet in a bar for the first time.