Responses
this is taken from the translator site; they actually explain it under their chapter translation: “zhan zheng xi is his original (full) name.... single letter name like ‘xi’ would be for really close relationships, often romantic. placing ‘a-’ like in ‘a-xi’ is for cute endearment”
so basically they both started blushing because he called him by a nickname used used exclusively for romantic couples (who are that close). hence the embarrassed gay blushing to their elbows. adorable, right?

can somebody explain why they reacted like that to "Xi"