I think Hifumi liked/was interested in Miike before he got the letter and that, even knowing the letter wasn't for him but for his sister, he decided to take advantage of that misunderstanding to make Miika fall for him. That's why he tries to change topics when Miika tries to explain the truth about the letter to him.
actually hifumi is indeed in love with miike, he used the letter as a excuse for miike to go out with him a first time, at the end of every date he stops miike from confronting him and invites he again to another date, his plan is to keep doing this until miike finally falls in love with him (ok this got confused but hifumi isn't evil, he's just desperate to date the boy he likes)
ooooh I understand, thanks!
Is he trying to bully Miike acting like he likes him? I’m confused