He is only mentioned in passing and not by name, as part of Hayama's clique.
He is one of the victims and also a suspect in the distributing of numerous chain messages that are spreading around the school. One of the messages claimed that he is a "three-timing douchebag". He ends up grouping with Ooka and Tobe for the workplace tours.
He is seen hanging out with Hayama after class.
In Ebina's culture festival play "the little prince", Yamato portrayed the king. He also happily carried props around for the show. For the concert finale he played the bass in the volunteer band.
During the Athletic Festival, Yamato is very serious about winning for the red team. He listens to Hayato's orders without hesitation during the boys bo-taoshi event.
Yamato and Ooka go with Tobe when he requests help with confessing to Ebina. They laugh the entire time and don't take it seriously. During the Kyoto trip Yamato is often seen hanging out with Hayato and his clique.
He accompanied his clique members to the spot where Tobe planned to confess to Hina and was greatly shocked when Hachiman confessed instead. After Hina's rejection of Hachiman, Ooka and Yamato console Tobe.
During the judo competition Yamato forms a team with Ooka and Tobe.
It's mentioned that he and Ooka chose the Humanities/Liberal Arts pathway for their third year. He is briefly considered by Hachiman as Hayama's "Y" crush.