Caste Heaven
Ch 11-14 is my favorite couple. Also appears in the ‘Behind the game’ in volume 3 (page 175), and in volume 6 (page 175). And also in the last chapter! (hinted at in graduation, and 10 years later in page 182) Volume 4 Side Story (Ch 175) has the buff bottom and the tall gloomy top. Volume 3 Side Story (Ch 147) and Volume 4 Behind the Game (Ch 199?) focus on the small x huge guy. Ch 15.7 has brief scenes of most couples.
Ibitsu no Kakera
Prequel: ‘Ibitsu na Kakera 14 Sai’ (The younger brother’s POV) Link: https://www.mangago.me/read-manga/ibitsu_na_karera_14_sai/ Sequel: Chapter 6 from ‘Rakuda Tsukai to Ouji no Yoru’ (also features the characters from ‘Kusatta Rasen’ and ‘Ienai Itami o Daite Iru’) Link: https://www.mangago.me/read-manga/rakuda_tsukai_to_ouji_no_yoru/bt/239040/Vol1_Ch6
Warehouse