Shinomiya is still around, just doesn't play a big role. She stays with the school post-apocalypse (Chapter/Volume 5 of the webnovel is the start of the apocalypse).
Ushikura and her family joined the Star Mother cult, her little sister is working as a liaison between him and the cult, but we haven't seen Ushikura since the apocalypse, we just know that her family went with them instead of joining with the school or any other faction. But we have seen a Hathor with the Star Mother Association, who seems to know Utamaro.
Certain cards have the ability to use kin summoning skills of one type or another, like the Kappas summoned by the Water Tiger in chapters 17 and 18 of the manga. Certain cards also have access to Kin Reinforcement skills, if a card meets a certain criteria, they can upgrade their skills to "True" versions of their skills.
Here's the MTL of what Renge says about the True Kin Reinforcement skill.
"...By consuming human souls, you can permanently transform your family into true family members."
"It seems the method of retrieving souls differs depending on the race, but I understand that if you use a human soul, you can create a true kinship. In my case, it seems I can only retrieve the souls of devout believers, either within my line of sight or through the temple."
"When creating a true kinship, if the god has authority over souls, such as the underworld, it seems possible to leave behind memories and personalities from the previous life."
Utamaro's thought: "...Perhaps there are people who would rather be reborn as True Kin than as humans."
So if a card uses Kin summoning, has True Kin Reinforcement, and has human souls available, they can reincarnate a human as the same type of monster that they can summon, creating a new card under their control, rather than the master who thinks they control the card.
Here's the MTL of when Utamaro meets the Hathor at the Star Mother Cult:
The first one is Hathor dressed in a white knit dress with a shawl draped over her shoulders.
The other was Sekhmet, a goddess with the head of a lion and the body of a voluptuous woman, clad in a black biker jacket and leather pants.
As expected of someone who advocates for the improvement of the human rights of cards and monsters, the cards are apparently dressed in human clothing.
Thinking about this, I casually looked at the two figures and was suddenly struck by a strong sense of déjà vu.
The object of my déjà vu is Hathor. It's definitely the first time I've met her, but I feel like I've met her somewhere before... Is it because she has somewhat Japanese-looking features, unlike Hathor? [NOTE: He's referencing another Hathor he knows here, this isn't communicated through the MTL well.]
Hathor smiled at me as I stared intently.
For a moment, I felt as if that face overlapped with someone else's face――
Also, Hathor is associated with cows and cattle, I believe all the ones in this series are beautiful women with cow horns. And Ushi means cow... so... yeah... I feel like that was another hint.
