Responses
The exact description doesn't quite match, but this reminds me of eighteen's bed?
School life
Brown haired MC who has a toxic crush on someone else (also brown haired), describes it as a foolish illness, often had to cover for him as you said.
ML has black hair + blue eyes and they aren't exactly in the same friend group? But they do end up basically attached at the hip for a while.
It's also a korean novel adaptation
I only began to the story but sadly can't recall the name (I wasn't logged in at the time). It starts with the mc wanting to be friends with a browned haired character, who he has to lie for to provide a cover about the brown haired guy fooling around with girls. The mc refers to his love as an illness for the brown haired guy, but I think the actual ml had dark black hair and blue eyes who likes him and is in the same friend group. It's a Korean manga. Sadly that's all I can recall, if anyone can find it you'd really help me out. (づ￣ ³￣)づ