The demi-ness of the girls all have some sort-of scientific explanations:
Hikari has a genetic mutation that stunted production of certain pigments in her body, This would explain both blonde hair, amber irises since those are under-pigmented version of Himari’s look, and haemoglobin is also a stunted pigment which resulted in her vampirism as a way of compensating it.
Yuki is born in a family that lived in cold places for ages. Her yuki-onna properties are the results of hundreds of years of natural selection and evolution favoring people with the best cold tolerance.
Sakie is a hyper-female, that is, her DNA has more than 2 copies of X chromosome. This does carry the implication that her child will always be either succubi if female or sterile if male.
Machi has a mutated gene that encodes an enzyme that can result in a clean line of quantum entanglement that bridges her head and her body. The three-Dullahan rule is enforced by the specifics of the quantum mechanics involved in those enzymes, but it does run in the family.
Given the settings, the accelerated genetic mutation can be chalked up to the nuclear disaster releasing excessive amounts of radioactivity into the soils of Japan.
A projected 5-way graduation as the end of this series:
The three girls graduated from high school and are all enrolled in college as undergraduates. Special mention to Mochi who is heading straight to a PhD in theoretical physics under Souma.
Takahashi’s series of research papers on demis based on the four demis around him was accepted and published, earning him a professorship at the college.
Sato decided to follow the steps of Mochi enrolling herself as a postgraduate with the intention of learning more about herself.
The final "still life" image of the end credits gives a "five years on" forecast-
The objects are:
Four books, colors-linked-to-characters as established in the end-credits: yellow (Hikari), blue (Yuki), green (Macchi), and purple/fuschia (Sakie), and
laptop (Takahashi-sensei, since he is the only person to consistently use one for the entire series.
The purple book is laying partially atop the laptop - Takahashi and Satou have a long-term-committed relationship or are married.
Confirmed. In Chapter 68 of the Manga, Takahashi asks Satou to be his girlfriend, and they confess their love for each other.
The green book is as close to the laptop as possible without laying atop it, and seems to be actually touching it - Macchi is a "sister wife" to Takahashi and Satou. Satou's power has been mellowed by her relationship with Takahashi, thus increasing her confidence in her ability to control harm, mellowing her, and allowing her to (in spite of the library tantrum while advising Macchi on her lovelife) allow Macchi in.
The yellow book is at a close distance from the laptop, leaning on a backdrop, but still between the far ends of the green and purple books - Hikari is quite possibly the continuing catalyst in the T/S/M relationship, and was probably the one that (intentionally or not) pointed out to Sakie that Macchi would make a good and compatible third. She remains a good friend.
The blue book is at a similar distance to the yellow book, but not between the purple and green ones. Yuki remains a good friend, but is not really involved in the T/S/M relationship.
