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Shougo speaks with a "country" dialect, one different from standard Tokyo dialect. Some people in Japan consider other dialects to be unpleasant or weird to hear, not to mention they have different words for the same things amongst dialects. The author wanted realism in that if Sho was raised with his gramps and gram he would speak that way. Its like if a story says Person X was raised in Missouri and then moved to Maryland, the Missourian would have an accent that the Marylanders would not be used to, its just that in Japan they are more strict about learning/using Tokyo dialect when in Tokyo
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I have to make a correction, I think Shougo actually uses Gyaru (Gal in English) language, which is like when young adults uses all those txting acronyms and slang instead of speaking actual words. Gyarus and gyaru language is pretty frowned upon and Gyaru's are generally girls (hence why its called 'gal'). So his speaking that way is off putting. Still in-line with what I said about standard Japanese being preferred in areas without dialects
I don't really understand the point of Shougo speaking like that. Its absolutely adorable but was there a purpose plot-wise? Or did the author just add it in cause they felt like it? ( ̄へ ̄)
IDK, its just been bothering me while I was reading.