What would you do if your childhood friend were to die, just for a spirit to possess and reanimate his body, living life like nothing ever happened? This interesting question is what The Summer Hikaru Died poses as it's guiding premise. A thought-provoking what-if that opens an array of doors to possible moral dilemmas, emotional predicaments and interesting mysteries. Unfortunately, The Summer Hikaru Died fails to deliver on any of those fronts. Instead, the narrative never expands any deeper than what you might see in a generic, romantic drama.

The plot does not experience any meaningful progression past episode 2. Sure, the backstory of Nonuki-sama gets explained in great detail, which you might find either nonessential or mildly interesting, depending on your edginess. Other than that, the far more interesting aspect of the premise is the psychological aspect of it all. How do you cope with such a sudden revelation in your personal life? How does this god possessing the body of your diseased friend, who is very much contempt with murdering anyone who finds out his secret, manipulating you emotionally into keeping quiet? Unfortunately, all answers to these questions are incredibly shallow. Yoshiki, though indeed having one or two doubts when Hikaru tries to kill him or the people around him, is simply in love with Hikaru's body too much. This statement is equally correct for basically any other character that finds out Hikaru's true nature. Rie Kurebayashi, who is the first person to severely warn Yoshiki about the consequences of his negligence, at some point also changes her viewpoint, sharing dinner with him, now actively searching for a solution to Yoshiki's and the monster's messed up love life. This is quite the letdown, as any possible authority that could persuade Yoshiki into doing the right thing completely vanished at that point. I just want to further note that not only has Nonuki-sama murdered thousands of people, but an Impurity has also killed Kurebayashi's child because she opened up to it too much. Why she ever would try to actively garner sympathy with Hikaru and try to find a solution for him then, is truly a mystery (that's a sarcastic statement, it simply doesn't make sense for a sensible human being in their right mind).
A similar assessment can be made about Asako Yamagishi. Not only has she figured out Hikaru's true nature as a monster, she was also almost killed by him. Any of those two instances should be enough to run far, far away. Yet, she still completely forgives Hikaru.

As a last note on the story subject, the entire B plot with Tanaka could've been completely scrapped and nothing would be different in the overarching story. What he accomplished in 12 episodes of runtime:

1. He erected a barrier that gave Hikaru ouchies for about 10 seconds
2. He had an incredibly important discussion about his hamster with Yoshiki
3. Goes on to have a beach party with Hikaru and Yoshiki at the end

The most meaningful thing he did was cutting off Hikaru's head. At least you would think that, would Hikaru's head not get sewn back on 20 seconds later, the screen fading to black and Hikaru and Yoshiki just appearing in a hospital all of a sudden. And what did Tanaka do in that time? Stare at them until they got taken to the hospital? The only explanation for this is that Tanaka's true goal is not to kill Hikaru. But that would also mean that Tanaka's true goal is to cut Hikaru's head off, then stare at him for 30 minutes 'till he regenerates. Pick your poison.

On a bit of a different note, I want to point attention to the show's frequent aversion of respecting the viewer as a capable observer. There are some nuanced story bits in The Summer Hikaru Died. However, every single of those nuanced story bits gets handed to you on a silver platter. Let's take the biggest story nuance of the show as an example and analyze it in that regard - the romance between Yoshiki and Hikaru. This aspect is not entirely clear from the start. Sure, you could speculate from the little nuances in the show right from the beginning, but in a best case scenario, you would pick up on the little clues over the entire runtime of the show, then form a complete picture at the end. This is not what happens here. The show does not recognize that you have this ability, and just outright hands you all the answers. On the topic of Hikaru's and Yoshiki's feelings towards each other, the show completely confirms this in the most anticlimactic way possible when they talk about someone from their village being shunned for not being straight. The anime really wants to sell you this aspect at that point. This is not the only example of this happening, though. On a slightly less important note, you might feel clever for spotting that the progressive darkening of the spot on Yoshiki's arm symbolizes the growing affection they have for one another and Yoshiki's progressive mixing but oh wait - they literally tell you that 10 minutes later. Or how Tanaka is blind, but is able to see with the glasses, which they felt they needed to actively explain to you, not trusting you to pick up on this already obvious fact. This is nothing that would greatly inhibit your overall enjoyment alone, but combined with many other grievances you might have with this show, it adds up.

Finally, before you send me messages of what a dullard I am despite no one even reading this far, I wanna bring attention to the art and animation of The Summer Hikaru Died, because it is truly not easy to watch. First of all, there's the extremely dull and bland character design. The main 4 characters are... fine, if I'm having a good day. Every other character is a complete blank slate with no memorability whatsoever. In the following image, Yuuta Maki and Yuki Tadokoro are depicted, who are the most important side characters in this show. However, not even they have received a better fate than being short haired guy #1161 and pigtail girl with a clip in her hair:

Now, I will obviously talk about the elephant in the room - the CGI. I'm not intrinsically opposed to all forms of CGI. I have seen anime that handled CGI with as much grace as they possibly could, though 2D animation would have been better regardless. However, nine out of ten times, it's absolute failure, and in The Summer Hikaru Died, we see that exact 90% case. Instead of explaining why these scenes look so bad, I'll just show some of the worst offenders, because anyone with at least one eye will be able to tell why these scenes look horrid:

also, pay attention to how many of the same models with maybe slightly different hair are present in the upper image.

this exact scene is shown multiple times throughout the anime, with no variation. If you do something like that already, maybe make sure it doesn't look completely cheap.

Takeaways

Despite an intriguing premise, The Summer Hikaru Died fails to expand on the initial idea in a meaningful way. Many of the characters that should've served as an opposition to Hikaru failed to accomplish exactly that, despite some of their backstories - and honestly human decency - suggesting otherwise. The anime does not value your ability to recognize details on your own, instead taking any nuanced deliberation away. Certain characters' roles in the show are questionable as well, as you're left wondering what their existence contributed to the story at all. The final nail in the coffin is the subpar presentation of it all, with bland character designs and an abundance of CGI animations that do not look appealing whatsoever.
As it stands, I cannot agree with the generally positive reception this anime has received and do not recommend watching The Summer Hikaru Died.