She became friends with her after Rikka temporarily got rid of her syndrome. Kazari then develops feelings for Rikka in secret.
Yuuta's relationship with Rikka puts him at odds with Kazari, even mentioning Kazari as his rival. Still, Kazari holds a friendly behavior towards Yuuta, she agreed to help Yuuta improve his relations with Rikka, arranging a date for the two (which also delighted Kazari herself, as if she was the one dating Rikka)
Kazari was beautiful enough to get Makoto's attention, he ranked her second on the cutie poll.
Nibutani was the one who originally suggested Rikka should ask her for relationship advice.