Yuzuru has the most polite language that I have ever seen. I haven't learned Keigo yet but I think that that's what he's using. I've tried my best to make him equally polite in English.
Mitsuru has a verbal tic where he tends to end his sentences with daze~ instead of the usual desu/da (Dash dash daze~ is one of his favorites). I've resorted to adding a 'yanno' every time he does this in Japanese.
Subaru and Makoto have this "Comedy Duo Act" thing that they like to do, which is based on Manzai. Basically, Subaru is the ボケ ("Boke" literally "airhead" but translated as "the funny man") so he's the one who acts dumb or says the funny things, while Makoto is the 突っ込み ("Tsukkomi" literally "person who interrupts" but translated as "the straight man") who corrects Subaru on his foolishness or ends the joke. So you'll see them referencing this a lot, especially talking about setting things up or whatever else.
Summer Lesson - 1
Summer Lesson - 3
Summer Lesson - 5
Summer Lesson - 8
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