This is not something that KyoAni (Kyoto Animation) has never done before by any stretch. In fact we are now accustomed to KyoAni changing the anime adaptation from the source material. To reiterate, the source material (the novel by Ayano Takeda, notably a young female) has Kumiko and Shuuichi end up together. Most would agree that this revelation that Kumiko loved Shuuichi came out of nowhere and almost, at least to me, seemed like the result of pressure from the publishing company. There is some confusion as the author tends to write everything like a yuri smut and the light novel even has these scenes powering the Kumiko and Reina pairing.
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Dialogue like this really makes one wonder. I think we can agree that this sounds like a line taken from a yuri smut.
Let us note, however, the differences between the anime and the novel in the festival scene with Kumiko and Reina, which takes place in episode 8 of the anime. In the novel, Reina runs her finger down Kumiko's cheek. It is a rather flirtatious gesture but at the same time quite platonic and plausible between friends. However, in the anime we notice that Reina actually runs her finger down from the top of Kumiko's nose down, eventually touching her upper lip. Lips hold innately sexual connotation that completely changes the dynamics of this gesture which seems now much more affectionate and intimate than in the source material.
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The face fingering scene in question. At, least to me, this action does not seem heterosexual or even platonic in nature.
Another such change was the addition of the duet they played which was not present in the source material. The title of the piece they played is "The Place Where We Found Love" and possibly refers to the location where they hiked and played this piece together.
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The Tokuyama Bhudda Lookout on Daikichi Mountain (仏徳山(大吉山)展望台), the real location upon where the scene is based upon.
Disney was well known for placing hidden meaning within backgrounds or areas where the audience does not have their attention drawn to. Often they are trying to convey an idea or hidden intentions that a keen viewer may pick up on. We notice in Euphonium, on the scene upon Daikichi Mountain that there are two stars visible in the night sky.
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The two stars in question.
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Another angle to show the authors' intentions place these two stars in the sky.
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Comparable graphic taken by the Hubble Space Telescope to show that I'm not just talking out of my ass. The two stars with the greatest luminosity are Orihime and Hikoboshi.
The festival that is occurring during this scene is called Tanabata, otherwise known as the Star Festival, which celebrates the meeting of the deities Orihime and Hikoboshi (represented by the stars Vega and Altair respectively). Taken from the Wikipedia page for Tanabata; according to legend, the Milky Way separates these lovers, and they are allowed to meet only once a year on the seventh day of the seventh lunar month of the lunisolar calendar. It is unlikely, if not impossible, that only these two stars would show in a night sky. They are purposefully having attention drawn to them. One could speculate parallels between the legend and the scenario taking place in the show. I'll leave that as an exercise for the reader.
It is perhaps unwise or misleading to be looking for connotation such that it fits the conclusion that I started with. I admit to my biases that I want Kumiko and Reina to engage in a gay romance. I ripped these images and many ideas off of /u/ and this is but a compilation of my thoughts. You may not be convinced, this is KyoAni we are talking about. In that case I will leave you with this.
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Before and after they played "The Place Where We Found Love", just the two of them, under the night sky where two lovers meet, inching closer to each other.
>inb4 all my speculation is wrong and hetshit ending
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