If nothing else, Hikaru is definitely acting like a teenager at this stage of the series. After Sai's disappearance, he lashed out and went off to search for him wherever there may have been a hint of him. With that falling through, and with a couple of episodes of mediocre filler material with other characters and flashbacks, he's been pretty somber and listless as he just attends his classes and doesn't interact with friends and colleagues. Keeping to himself as a punishment for what has happened, he's causing quite a bit of concern among people, be it his friends and Go colleagues to his mother as well. His mood has changed significantly and it's definitely very appropriate for his age and situation.
While most are concerned, some are much more vocal about it. An encounter with Fujisaki has him revealing that he may indeed quit the club, but it's when Mitani comes along that it gets heated. Mitani can't stand to see what Hikaru is doing, nor understand it, so he lashes out at the way he perceives that Hikaru is looking down at him and the others in the club for "just a tournament." It does have some impact on Hikaru as he remembers back a couple of years ago when he was so eager and was trying to get Mitani into the club so they had enough members. Even from a distance Hikaru is earning some scorn as there's a very cute moment with Tsubaki at his construction job where he grumbles about what Hikaru is doing. If he sees him, you'd expect he'd be even more in Hikaru's face.
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