The "elite student in music room 2" turns out to be Haruki's delinquent classmate, Kazusa Touma.  With his characteristic pushiness, he tries to get her to join the light music club.  But his pushiness backfires, and only makes her more annoyed.  Setsuna gets irritated... no, involved, and approaches Kazusa secretly.
It was your joining his band.
Es war dein Beitritt in die Band.
Fue que tú te unieras a su banda.
Quiere hacerlo porque te uniste tú.
Il a décidé de le faire lorsque 
tu as rejoint le groupe.
Foi você ter entrado na banda dele.