I could be wrong but I think it's more of an intense interest in what the letter is. His interest is seriously piqued by what her brother says. LKO
I thought that too but Kai insinuates that the letter would be embarrassing and I doubt that Ares would know or care about a letter ╮( ̄▽ ̄)╭ like why would Kai want to show her bf the letter, what's in it ya know
Sorry if I'm a bit slow, but here... is Ares jealous/mad about the possiblity of Alicia having a boyfriend?
http://www.mangago.zone/read-manga/living_as_the_tyrant_s_older_sister/nto/mpn_chapter-2214050/pg-5/