Awesome Music: The fact that the ending theme was given a cover by Maru's and Kiruko's actors says a lot about its appeal.
Broken Base: The fan base is divided on how the situation regarding the Chief of the Water Filtration Center Inazaki Robin was handled in the “Inazaki Robin” arc. On the one hand, had Maru murdered Robin, that would’ve put an end to the latter’s vile surgical experiments, including hooking up body fluid transfer tubes between a Man-Eater and a woman missing several of her limbs while still conscious. Having left Robin alive and unrestrained gave him the opportunity to steal equipment from the Ministry of Reconstruction, flee with his guards, and disappear to unknown parts where he’ll continue to perform gruesome experiments on innocent people. On the other hand, Kiruko managing to convince Maru not to murder Robin for vengeance meant Maru hasn’t resorted to cold blood measures to eliminate evil people. While it might seem like Robin had it coming, Maru is an strong and fast teenager who can take heavy hits from adults and push them around like they’re nothing. If he of all people started to get a taste for bloody vengeance, he could turn into a vicious Knight Templar in a lawless post-apocalyptic setting, one where few people could stop him.
Captain Obvious Reveal: In Chapter Thirty, it’s the fan base learning that the Director is an evil and vile woman behind her facade of a nice old lady. It’s practically spelled out in the first chapter regarding how she tells Tokio the outside world is hell that something is wrong with the old lady.
On the other hand, at this point in the story, the reader doesn't know that the great tragedy hasn't happened yet. The Director seems to be referring to the post-apocalyptic world we see Kiruko and Maru inhabit as hell, which seems fair.
Catharsis Factor: In the “Inazaki Robin” arc, there is the time when Maru confronted the Chief of the Water Filtration Center aka Robin for raping Kiruko. On the one hand, many readers found it satisfying to watch Maru throw Robin repeatedly at a wall when Maru cornered Robin from escaping, giving him a taste of the kind of humiliation that comes with being harmed by someone and not being able to do anything about it. As a result, said readers became frustrated when Kiruko showed up at the last second to convince Maru not to go through with murdering Robin. However, other readers noted that the way Maru tormented Robin had a sadistic feeling to it, a far cry from Maru’s normal approach to fighting people, which worried these readers with the fact he was becoming vicious and committing excessive amounts of violence to punish Robin. Maru also came close to ending Robin’s life for vengeance, so considering for that moment Maru’s personality warped from a Nice Guy into a potential cold blood murderer it’s understandable why Kiruko - a Martial Pacifist herself - was terrified at the idea of someone she cherished becoming a frightening executioner. When she managed to talk him down from murdering Robin, some readers felt immense relief that Maru didn’t stain his hands with death and blood, but unfortunately for these readers he hasn’t quite snapped out of his rage and secretly still wants to end Robin’s life if given the opportunity.
Creepy Cute: Asura from the “Nursery” plot line is this. While most of her body is that of a normal teenage girl, her head is alien-like and stands out from the other children. Despite this, she was one of the most benevolent children with healing powers who also was close friends with Kona, the last remaining child besides her in their respective class. Having committed suicide for unknown reasons devastated Kona, but she told him it was because of an important destiny she had to fulfill, leaving the fan base wondering if she’s truly dead since Mimihime can see a silhouette of Asura near the ceiling of the nursery. Some even believe she might be the true Big Good of the series, one who has yet to reveal her true reason for hanging herself.
Ensemble Dark Horse: Mimihime from the “Nursery” plot line is considered this by the fan base. Besides being a Nice Girl whose kindness is genuine, she’s the one who helps Shiro come out of his shell and uses her precognitive powers to try and make the best futures possible (such as not holding hands with Taka when Anzu was nearby). This continues into the Post-Disaster plot line as she ends up being the bandaged patient Hoshino that Dr. Usami, aka a grown up Shiro, continued to watch over. At the end of her life, she never hated him for the numerous surgery procedures and medical equipment being hooked up to her, knowing that had he not taken such extreme measures she would’ve died and become a Man Eater. When Maru showed up, choosing to grant her a Mercy Kill by using his “Maru Touch” ability at Shiro’s request to end her suffering and ensure she didn’t become a Man-Eater upon dying, she typed onto her communication screen her final parting words to Shiro: a declaration of love to him, making it one of the saddest points in the arc if not the entire story.
Fan Nickname: "Tsuyu/Froppy" - Kuku's nickname due to...well...looking and sounding exactly like Froppy from My Hero Academia. Even her abilities are nearly identical (i.e. frog-like) to Froppy.
Jerkass Woobie: Juuichi, aka “Seed Pig Number Eleven,” is this in a nutshell. While he first tried to rip off Maru and Kiruko with information they would’ve found on their own for their cash, he later presented himself more sympathetically when he revealed the story about him being a survivor from a cult of misandrist women was true. He wanted to know if his son was still alive, but the only way to do so was to somehow approach the cult, which was impossible for him to do since the women would cut him down. When Maru and Kiruko agreed to help Juuichi, he drove them in his vehicle all the way near the place of the cult. But after it turned out the cult had fled because a Man-Eater attacked them and took over the building, they discovered some of the “Seed Pig” men had survived and established their own community not too far away. Juuichi reunited with his son named Juugo who was still alive, plus Maru and Kiruko helped the community deal with another “Man-Eater attack” that was in reality Juuichi’s son, who was able to unconsciously channel the same exact ice abilities the Man-Eater in the cult’s building had used. Afterwards, Juuichi and the other men thanked the duo, Juuichi having decided to give them his vehicle so they could use it from then on. Later, Juuichi murdered one of the men while the others were gone, whose voice Juuichi recognized as the one whom tipped off the cult that Juuichi and the two women who decided to help him escape with his son were running away. While Juuichi admitted that he didn’t know the full story about why the man did this, even admitting it was possible he may have been a traitor for running away with the women and his son while leaving behind the others, he still couldn’t forgive the man for being the reason why Juugo’s mother was hanged to death, along with the other sympathetic woman who helped him escape. He then took his son and fled the community in another vehicle the group had, leaving behind the others in order to escape retribution for murdering one of their own. In short, Juuichi was a complicated man who went through hell and back to save his son, but did many morally questionable things to accomplish this.
LGBT Fanbase: Word of mouth regarding the casual lesbian kiss early on in the series, and especially Kiruko's Ambiguous Gender Identity situation, has garned the series a significant LGBT fanbase. The fact that Maru is still attracted to Kiruko despite learning the true gender has made the shipping of the main duo incredibly popular.
Realism-Induced Horror: In the “Inazaki Robin” arc of the Post-Disaster plot line, the Chief of the Water Filtration Centre, Robin himself, is considered to be one of the darkest and vilest characters of the series by the fan base, yet also one of the most unexpected. The Chief was seen by the town’s citizens as a smart and benevolent member of the Ministry of Reconstruction who used their good looks; suave personality with charming others (especially women); and important role to avoid being detected as a criminal preying upon innocent people, which mimics some forms of real life criminal behavior. In a series filled with supernatural horrors, it’s telling that the actions of Robin raping Kiruko, a woman who trusted him with her life since he had been a Big Brother Mentor to her in the past comes off as more terrifying to a great deal of the fan base rather than being “eaten” by a giant animal-like monster.
Tear Jerker: Mimihime's death, she never blamed Shiro/Dr.Usami's for keeping her alive despite the fact that in her final moments she was pretty much a living torso. Her last words after Maru Mercy Kills her is herself thanking him for all the good he's done for her over the years and giving her a dignified death, which ends with a poignant: "I love you", naturally Shiro breaks down and cries. The anime version of the scene takes it even further thanks to the added soundtrack and direction. Masakazu Ishiguro tweetedthat the episode made him cry, even though he'd written the scene originally.
Trans Audience Interpretation: Kiruko, a guy who's brain was transplanted into a female body to save him, but has since fully adjusted to being a girl and does not seem to mind people seeing her as a woman at all. Needless to say, it's easy to read her as a trans woman, a trans man, or anything in-between.
Reading her as a trans woman, she is fully comfortable being treated as a girl, and seems to think of herself as one at times even. The first time she claims to be a guy is when trying to dissuade Maru from liking her, which could be read as similar to internalized transphobia.
Reading them as a trans man, Kiruko is quite literally a man in a woman's body. Hard to get more transmasc than that.
For in-between, she could be read as being comfortable as both a man and a woman, identifying as both either concurrently or interchangeably, and has gotten used to being a girl over the last few years
Visual Effects of Awesome: The anime boosts movie quality visuals and incredibly fluid animation, a surprisingly rare sight for an adaptation of a relatively not-well-known seinen manga. Some of the segments, such as the Maru's fight scene with a couple of gang members in the first episode, would put even a Matoko Shinkai film to shame.
that the episode made him cry, even though he'd written the scene originally.