Kazumi, or rather, Michiru Kazusa's relationship with her grandmother.
Kaoru is made up of a number of these playing with her Cool Big Sis status.
Chapter 10's reveal that she didn't wish for a healthy body, she wished that everyone who had been harmed in the soccer game where she was severely injured was healed. She wasn't thinking about her own injury, but she felt incredibly guilty for the girl who did injure her. Apparently the girl was being harassed for accidentally crippling Kaoru to the point that she tried to kill herself. That Kaoru actually contemplated suicide herself showed she had no ill will towards the girl, and probably never even thought of her until she saw her unconscious body being brought to the hospital. It's also one of the few times someome makes a Selfless Wish with no regrets
Chapter 13 and 15 demonstrated that Kaoru is the only Pleiades who doesn't consider Kazumi to be expendable, and loves her for who she is, not that she's a clone of Michiru Kazusa. Kaoru actually tries to warn Kazumi to get away and start over because the Pleiades will indeed try to make another Michiru clone, and Kazumi deserves to live.
Chapter 20 reveals that each of the Pleiades actually was looking forward to Kazumi's existence as a blank slate and new person. They all had gifts to welcome her. It was only when Kazumi began to seem unstable and Satomi snapped that they were forced to make a choice between themselves and Kazumi.
In Chapter 19, after Saki became a witch and decapitated Mirai, it seems as though she's about to do the same to Kazumi. Kazumi herself is in no condition to fight or even run away, as she is on the verge of turning into a witch herself and can't expend any energy; Kaoru and Umika realize that they can heal Kazumi with a Grief Seed, but don't have any. After hearing the ringing of Kazumi's bell earrings, Saki's witch suddenly stops and lets out a cry, which makes the witches around her suddenly turn on her and rip her to shreds. Soon nothing remains but a Grief Seed; Kyubey, from a distance, remarks that Saki's witch has the disposition of "sisterly love." All Kaoru and Umika can do is tearfully say "Saki... we know, we know..." before healing Kazumi.