According to end song of season 21 to 22 detective conan. There is one girl that appears and seem to be liking Shiniji, then she start to be upset seeibg run stay with Shiniji. I believe the first girl that have her secret love to Shiniji in high school is Sera Masumi.
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Oh, what is the reason why i though of this?
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I just re-watch conan and found something very interesting at anime episode 743 [691 jpn.] At almost the end of the episode there is one moment that Conan talking just exactly like shiniji and its made run take a very close look to Conan. Still, those are not the main point. After that , Run notice that conan have a wound on his arm, Run ask Conan for her to help him with the wound...and then SERA LOOKING BACK TO RUN TAKING CARE OF CONAN(WHICH I THINK SERA KNOW CONAN IS SHINIJI) THEN SHE CRIED!!!!THATS RIGHT, SERA CRIED LOOKING AT RUN TAKING CARE CONAN(SHINIJI) IN THAT SPECIFIC SITUATION. Im still belive it like 7/10 so i want to hear out about you guys's though, too. If i wrong it is my apologize. And sorry for wasting your valuable time.
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Thanks for reading.