The central relationships are platonic/familial.
I'll start with an idea with Shinobu and Kanae.
-I would have liked to see more about their sister relationship in the series, but with Kanae dead, it's difficult. That's why fanfics exist, to change history.
I wish Kanae hadn't died. They could have had her fighting Douma and he would have left her badly hurt, but alive.
-Shinobu finds her in time, they take her to treat her wounds, Shinobu does not want to leave her sister's side.
-Kanae remains in a coma for 2 years.
-Shinobu trains a lot, becoming a hashira. His main motivation: to be able to protect Kanae from all the demons.
-Every day she spends the greatest amount of time with her older sister, she talks to her, reads, asks her to wake up, to be with her and every night she sleeps with her, needing her comforting presence.
-Kanae can hear her, but she can't get out of the trance she's in.
-The day Shinobu goes to fight Douma (And all the other hashiras face the other upper moons), Kanae wakes up. When she finds out where her sister is and everything that has happened (Someone tells her) she runs to help and protect her, being her first instinct as an older sister. He arrives just as Douma injures Shinobu. Seeing this, Kanae becomes enraged, confronts Douma and protects her little sister.
-Both sisters defeat him.
-They both cry, Shinobu excited that her older sister is awake and Kanae happy to see her little sister safe. Of course, not before Kanae worried about Shinobu, checking if she was okay and if her injuries were hurting too much. They love each other more than anything and neither of them could see the other hurt.