Responses
It's sensei. Iin the extra there's a picture of the two main couples together after graduation I suppose :
http://www.mangago.zone/read-manga/nure_nezumi_tachi_no_koi/uu/cf203d7f927b9b0e489e385c7deba601_chapter-5/pg-2/
I guess the relationship took time to develop, so they didn't get together right away which is why Yuuji said things didn't change between them, but sensei accepted Yuuji's feelings there :
http://www.mangago.zone/read-manga/nure_nezumi_tachi_no_koi/uu/nure_nezumi_tachi_no_koi-chapter-3/58/
Wait who is a guy with yuuji?