I can't support the Vampire!Luna theory as there's no real hint towards that other than Naoto's sharp canines, but we got some evidence to back up the non-human/supernatural characters theory, both in-story and meta.
I just kind of threw "Vampire" out as a kind of blanket idea because some things about her made me make some loose connections. These are just things like her name, the fact that she seems to be mostly active at night (no I'm not talking about
that) / seems to have a hard time waking up in the morning, the change in her hair colour (that may or may not be natural), attracts men to her seemingly without trying and her pupils.
If anything, Naoto really fits the Vampire aesthetic more than Luna does as he's tall (by Japanese standards), handsome, attracts women to him almost similarly to Luna does men and the teeth. We can't comment on his eyes, as we've yet to see them in their full shape yet.
- Since Chapter 1 - Luna's DIAMOND SHAPED eye pupils - SHARP. DIAMOND. SHAPED. PUPILS. They can contract and expand, and Luna had them since the flashbacks, making them HIGHLY UNLIKELY to be Cat's Eye Syndrome, or contact lenses.
The eye shape is also something that is exclusive to predators (mostly night time) in nature as well. I think the closest thing to having that shape of pupils is the Fox.
- Since Chapter 5 - Kawashina's SHARK TEETH in full details - She's had them since the flashbacks, they're a true feature of hers as noted by Hiba in Chapter 17.1 in a flashback from their trip to the beach during Chapters 13~14, and she used them to draw blood when she bit Naoto in the flashback in Chapter 8.
There's also the part where she admitted that she's being dying her hair the entire time and the Blonde / light hair is her natural hair colour, which is something that is extremely uncommon (but not impossible) for a Japanese person, or most Asians.
Yes, with Hibas comment about the teeth, we now know that those are not some "artistic effect" and are in fact what her teeth actually look like.
Just going to point out that
WE STILL DON'T KNOW WHAT HER FIRST FUCKING NAME IS.
- Since Chapter 6 - Hiba's SNAKE EYES - She consistently had them since the flashbacks and are still consistent in the present, they too can contract and expand as seen in Chapter 18.2 when getting punished by Luna, and after Luna's done with her, the thin slits seen are her natural resting eyes.
Her overall appearance and some of her actions also lean into her being snake like as well (not in a bad way, and not in the western idea of what it means when you call someone a "snake")
- Since Chapter 6 - Naoto's UNUSUALLY SHARP CANINES - Hiba notes that his canines "haven't changed since Naoto was little", and him just accidentally biting down was enough to draw blood.
We also see them poking out
all the time when we get a close up of his face.
- Meta - Suga Atsushi's Isekai title - A majority of the non-human characters don't share the more round pupils that human characters have. - Human characters are consistently drawn to have rounder pupils in ALL of Suga-sensei's works.
This is also something that has stood out the entire time. If it was something that Suga used in previous works in normal situations just to give characters extra flair, it would be one thing.....
- Meta - Volume 4's exclusive bonus chapter (Chapter 20.5) - In the alternate world bonus chapter, Luna is depicted as a nun, making her SISTER LUNA, get it? Naoto delivers her medicine at her request. Why she needed a small crate-full of it despite being the ONLY PERSON IN THE VILLAGE CHURCH, we'll never know. She tells him not to visit her for a while, but when he did, Luna jumped him and it turned out that her being a nun, SISTER LUNA, is just a facade and that she, JUST LUNA, is a literal SUCCUBUS. - This could potentially be the author throwing the readers a huge hint on what the main story is actually about, with none of us readers the wiser, thinking this is just a bonus chapter with no real meaning whatsoever to the main story. Interpret the bonus chapter as you will.
It really is kind of hard on how to interpret that chapter. It could be relevant to the actual story and a nod to what's to come, or it could have just been a fun little short story.
On a side note, Chapter 17.2 page 18, the childhood photo retcons the very same photo from chapter 1, and it heavily nerfed the breast size of the "third girl" to match Hiba's own (Chapter 17.1 for comparison)... I don't know what else to say, but she's meant to be Hiba. We got all the important characters already.
It's not like I'm opposed to it being Hiba in the photo, but I need some solid confirmation on it before I'm sold on the idea, since their appearance could just end up being similar. Hiba being the third girl (and romantic interest) is actually something I'm completely fine with as I actually like her character more than Kawashina and she seems the most likely out of the three to actively pursue Naoto and shake things up.
Like, if Luna gives the OK for her to go after Naoto, I feel like she will actually do it and make the other two either accept their feelings and keep up, or be left behind.
I myself am currently leaning on the theory of
Luna having a dependency on Naoto, or more specifically, Naoto as "Her younger relative whom she treated as her real younger brother that played with her in the countryside up until 10 years prior to the beginning of the story". The evidence towards this theory just kept stacking up.
- From Luna growing up alone and suffering from loneliness and depression (Chapter 12, Chapter 22, Chapter 24.1).
- To her playing with her "younger brother Nao-kun" claiming it was "the happiest time of her life" (Chapter 12), with Naoto becoming a Living Emotional Crutch for her.
- Her not wanting to be separated from him (Chapter 4.5, Chapter 9, Chapter 25.2), further supporting the theory with Naoto being a Living Emotional Crutch for her, he is the only person that can make her truly feel happy.
- Her reactions towards others should any harm befall Naoto. (Chapter 8, Chapter 24.1)
- Her still seeing the shadow of the younger "Nao-kun"(Chapter 19.2), despite all the "progress" the two had at that point.
- And her tendency of wanting to play with Naoto the games they played in their childhood, a call back to the "happiest time of her life", where she played and had fun with "Nao-kun" (Chapter 9, Chapter 14, any chapters where she mentions wanting to hang out and spend some fun times WITH HIM SPECIFICALLY really, Hiba even had to point this out to Naoto by giving him a hint in Chapter 14 when Naoto thinks that he is not needed for Luna to enjoy herself).
There's so much going on and so much that's left out right now, it's hard to get a good grasp on things relating to her past and why she's so desperate to hold on to Naoto as a "little brother" and thinks that he only loves "Luna-nee".
Although one thing I have noticed, her apartment showed clear signs of the standard "depression room" and as the story progressed and she became happy, the state of the apartment also improved and she even went as far as to try and "help" with the laundry (snatching up Naotos underwear), showing that her mental state has clear signs of improvement.
I'm not going to lie though, I get the feeling Suga may have been talked out of the non-human / supernatural route and that's how we ended up with
Shounin Skill de Maoujou no Kouryaku wo Mezasu ~Isekai Ten'i Shita node Saikyou Item to Talk-jutsu de Ikinuku koto ni shita~. Which would kind of suck because it feels like a waste of all of these subtle hints towards some supernatural shenanigans and the visual character traits that they were all given.
Edit: Also going to point out that with all of the focus we see on Luna, her problems and their relationship problems, we actually don't even really know much about our MC himself. We haven't really gotten any information about his character and past either. We don't know about his life before or after Luna. We have no idea if he's ever actually been in a relationship in the 10 years before all of this.
