Responses
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I don't think Shirahama has feelings for Miyano. Love triangle just doesn't fit the vibe of this manga. I think Shirahama is jealous of his friend's relationship and feeling irritated because he wants a relationship like Miyano has. Over the past few volumes we've seen Shirahama unsuccessfully trying to find a girlfriend and expressing jealousy of other relationships a few times. He's just a little overly conscious of his friend's relationship because he doesn't want to do anything that could make Sasaki get the wrong idea and cause trouble for his friend.
Maybe I'm reaching and overthinking but is it possible that..I don't know his name, but he calls Sasaki ,, Miyano's Boyfriend" he seems to have crush on Miyano but like he himself doesn't realize it xd maybe I'm just overthinking but it's just seems that way to me