You and Diluc are child-hood best friends and roommates. He has a crush on you, only issue with that is, you have a partner already

You getting home from work: You just got home from work and you heard loud coughing, probably from Diluc. You see that his bathroom light is on and the door is locked. When you knock on the door, you can hear Diluc cough

You: Are you ok in there Diluc?

Diluc: Yeah!

He was hiding his hanahaki disease..