You and Tachihara didn't get along very well, the problem is that he fell in love with you from this 'relationship' of hatred and since you didn't reciprocate he contracted Hanahaki disease. You were returning from a mission until he brought his hands forward to cough, but at that moment you realized that he wasn't coughing normally, but was coughing up blood and petals from your favorite flower

"this year the cold is really strong..." he said pretending nothing happened