These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Kachess' reaction to Yufina going missing is to search almost every country to find any form of evidence as to were she went, then disguise himself as a woman to sneak into the Duke's harem and kill the guy to save her. Talk about dedication!
While it has died down a bit, there was a small fraction of the fandom that insisted that Kayo Sudou was stressed out and overworked and just looking for someone to comfort her. This apparently justified her murdering three people out of spite and then killing the man she claimed to love because he didn't feel the same way about her.
While Allen isn't exactly the embodiment of evil, there are plenty of fans who like to ignore his less sympathetic qualities (looking down on/ignoring anyone he doesn't happen to like, killing his own adoptive father to appease his sister, etc.) and focus exclusively on his tragic woobie status. In extreme cases, they'll go on to demonize Riliane and blame her for the whole thing, despite the fact that Allen carried a lot of her underhanded deeds by his own choice.
Ear Worm: SO MANY. The "Lu, Li, La" part of the Clockwork Lullaby series is pretty much guaranteed to get stuck in your head.
Hell Is That Noise: The guillotine sound in Daughter of Evil and Servant of Evil, except in certain versions where it just sounds like a rope being cut, and other versions where it's left out entirely.
The gunshot in The Last Revolver.As well as the second one.
Internet Backdraft: Discussing who Michaela loved is a pretty good way to start an argument in some circles. Fanmade songs, such as Daughter of Green and at least two different songs titled Prince of Blue, get cited as evidence rather frequently in favor of either Kyle or Allen, something not helped by the fact that the official Daughter of White PV actually does seem to suggest Michaela loved Kyle (she appears to be hugging him back). On the other hand, information from books isn't as easily accessible, but they contain the clearest explanations as to Michaela's relationship, and Wooden Girl~Thousand Year Wiegenlied~ having her say she loves Clarith (but doesn't specify whether it's platonic or not). The result is a lot of confusion and attempts to sort out what's canon or not. On rare occasions, an argument against Clarith x Michaela will insist that Discount Lesbians or Heterosexual Life-Partners are in play, and the conversation sharply turns into a debate on whether or not that claim is homophobic.
Les Yay: After some years, fan speculation over whom Michaela truly loved - ranging from her having a true relationship with Kyle, as is implied by quite a bit of fanart, or actually being in love with Allen the whole time, which was input into its fanmade song - eventually ended up moot because regardless of whatever she felt towards Kyle, her true love was towards Clarith.
Also occurs in the songs where there are twoRins. In South North Story Rin and Riliane get very intimate with each other and in Screws, Gears and Pride the two Rins pretty much give each other the "it's not you, it's me" speech when they part ways.
In the trailer for Praeludium of Red, there's a brief image of Elluka and Gumilia looking awfully chummy...
Some translations of Capriccio Farce have the doll call Ma "my dear Sorceress of Time".
The Madness of Duke Venomania Manga adaptation makes Lukana Octo a walking magnet of these. She was good friends with Lilien Tanner (a baker), and bought her a beautiful bracelet to match hers. In fact, looking for Lukana is Lilien's motivation to follow the duke at first. There's also an scene where Lukana is hugging and talking to Mikulia about the duke/their situation. Mikulia's only answer is that she loves Venomania... and she also loves Lukana.
Gumillia's feelings for Michaela are compared to Michaela's feelings for Clarith. At first Gumillia vows again and again to protect Michaela, and even before they become human, she tracks down Michaela's location when she goes missing for two weeks; similarly, in both the novel and the song, Michaela emphasizes how much she wants to protect Clarith, and occasionally it's Clarith who wants to protect her. When Michaela tells her that she will remain as a human in order to be with someone important to her, Gumillia responds that she has also just decided there's someone important she needs to protect and will thus remain human for, with the heavy implication that it's Michaela ("if Michaela wants to stay as a human, then I also donít want to revert back into a spirit"). Later, Gumillia is compared to Clarith when she challenges her on who will get to take care of tree sprout Michaela; Clarith realizes that "Michaela is an important person for her as well", and various characters have referred to their romantic interests as important people.
The scanned images for the Conchita novel have a...interesting image of Platonic and Elluka together.
Irina deserves special mention here, after all she is pretty much the one manipulating EVERY single character (Not even Elluka of all people saves herself from this ...literally) behind the events of Story of Evil and the Pere Noel Organization, we could fill an entire article just listing each one of her schemes, including manipulating Prim Marlon the former resident Magnificent Bastard
Magnum Opus: Story of Evil. Servant of Evil in particular. It's the song that gained mothy his producer name in the Vocaloid fanbase, after all.
Prim of Story of Evil can cross this in one of several places, depending on whether you consider what happened first in the timeline or what the audience got to see first, but the most damning thing comes when the audience finally sees the whole picture: She triggered two decades' worth of international suffering and warfare, all for the sake of spiting one woman who died halfway through this plot.
Narm: Whenever mothy fails to pull off Nightmare Fuel or Tear Jerkers, it usually becomes this. For example, Allen's paragraph-long scream in the novels.
Tear Jerker: Many, but special mention goes to Servant of Evil, Regret Message, Daughter of White, Girl of a Miniature Garden, Flames of Yellow Phosphorus, The Last Revolver, Maiden of Wood ~1000-Year Wiegenlied~, Kept Waiting for a Response and White Brick and Black Mourning Dress.
A minor one where Germaine found out that the princess she captured in the Lucifenia Revolution is actually her foster brother whom practically grew up with and loved like family.
Meta Salmhofer, in Escape of Salmhofer the Witch. Growing up without parents and engaging in a life of crime and terrorism for a while, she had a change of heart after giving birth to twins, and resolved to protect her children from experimentation, only to get murdered and her children kidnapped by Eve. She mentions in her song that she's never been loved by anyone. Ever.