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.
Fanon: Though Mireille's brother is never named on-screen, there's a tendency to name him "Roland" in some fan works, which actually comes from a mistranslation of their father Laurent's name, which in Japanese wounds up sounding like "Rorando", which is identical to the pronunciation of Roland in the same language.
Also, the idea that Kirika is a Self-Made Orphan and killed her biological parents and/or the couple who was posing as her parents prior to the first episode is pretty commonly accepted.
Foe Yay: Lady Silvana, aka the Intoccabile, gives Mireille an excessively long (and open-mouthed) Kiss of Death. And beforehand, when they were children, she was veryobsessed with Mireille, traumatizing her for life. It's also implied that she has a general obsession with domination and control, and used the kiss to put fear into Mireille by confusing her and establishing dominance.
Growing the Beard: The show improves quite dramatically after the St Petersburg episode.
Much of Kirika and Mireille's prominent actions can be interpreted as such: ( Mireille not being able to kill Kirika after their "contract" was up as promised, even after finding out Kirika was the one to kill her family, Mireille killing her only remaining family member for Kirika's sake - the one who raised her after the demise of her parents, and the letter Kirika left for Mireille can very much be seen as a love letter.)
In addition to the Les Yay between Kirika and Mireille, there are also significantly less subtle examples between Kirika and Chloe and Mireille and Lady Silvana.
Narm: Altena's scene in front of the volcanic vent with her hair and skirt blowing around made more fans laugh than think it to be dramatic as intended and spawned comments of "Octopus!" and parody fan art of her imitating Marilyn Monroe.
The Woobie: Kirika practically holds up a sign that says "Please Hug Me!!" whenever she is crying. Chloe also gets in on the Woobiedom in the final arc, and even somewhat before that.