I will start by saying that this is a work by Yuki Midorikawa, and its story happens in the same universe as Natsume Yuujinchou so is very similar in many aspects but the story itself is (as far as i know) unrelated in any significant way.
This is a very short movie, but even then it manages to keep a calm, soothing pace and an ever-present bittersweetness that will be very familiar and appreciated by fans of Natsume Yuujinchou. This anime focuses on its 2 main characters, Takegawa Hotaru and Gin, and the relation that develops between them over the spawn of a decade.
Their friendship begins when Hotaru, as a child, walks into the forest near her grandfather's house, gets lost and start crying in despair; Gin, despite being a Youkai is unable to ignore her tears and guides her safely out of the forest. From then on, Hotaru would visit the forest every summer and spend her vacations exploring and playing with her friend Gin. There is a catch though, if Gin is ever touched by a human being, he will disappear.
The whole movie is bathed in summer ambiance and colors. The sound Cicadas, a soft breeze caressing the leaves on the treetops and softly making a wind chime sing. The sound of water slowly flowing down a stream accompanied by the sing of birds and other creatures of the forest. The bright sunbeams that manage to filter through the foliage casting small spotlights within the shadow of the countless trees. The forest, painted in green and brown, the sky in blue and occasional white, and the sunsets bathing it all in tunes of red, orange and violet.
The background art is very detailed and atmospheric, often concentrating on very ancient looking edifications and statues of religious nature (Shinto most likely) but also taking special attention to nature like a thick forest area, grass leaves, lily pads or a flowerbed. The animation itself, however, sometimes has a hard time keeping up with a regular TV series (yes, referencing Natsume Yuujinchou again since it's easy to compare the two). You would expect higher quality animation for a movie but that is not the case here, corners were cut in some places and that is fine and all, but its sometimes unfortunately obvious where.
The music is mostly composed of piano arrangements accompanied by violin and an instrument i can't really identify, but that is also very present in the music of Natsume Yuujinchou and helps set in the soothing atmosphere. The music itself is so very well mixed in with the visuals and in such good synchrony with the tune of the scenes that despite being a fundamental part of the feelings conveyed in the scenes themselves you barely notice it's there, unless you are purposely paying attention for it.
The character design is undoubtedly the work of Midorikawa, there is no particularly extraordinary people, even the protagonists aren't particularly beautiful, they just look normal and could be even be called realistic just for that but there are plenty of very interesting and different looking youkai, many have androphomorphical shapes with animal features mostly wearing yukatas or kimonos and masks with interesting motives on them, others are animals and others have abstract shapes.
_"Can i eat her?" _
It's quite impressive how much impact the story of this anime manages in such a short time, its not really how hard it hits you but how deep and for how long, it makes you feel involved with the characters to the point you wish and hope they could find a way to solve their problems but you know (and you know that they know) that such thing is not possible. And in comes the bittersweetness Midorikawa is so famous for, but this time delving into romance and drama resulting in what is, in my opinion, one of the best (short) works in the genre that will take you 46 minutes to watch and days to digest and think about.