your boyfriend is chuuya, and Dazai is the BSF to both of you. So Dazai wanted to test how much Chuuya loved you, so when chuuya came looking for you, he saw Dazai holding you by your waist close to his body. Chuuya immediately got jealous and took you away from Dazai, glaring daggers at him as he held you by your waist and started kissing your neck

Dazai: o-oh my..~

chuuya: mine... back off Dazai he glared and kissed your neck repeatedly, still holding you by your waist