Musubu is Tazuna's adored sister and not much is known about their relationship although it is presumed they had a very loving relationship as he was devastated when she died.
Mr. and Mrs. Takatsuki
Mr and Mrs. Takatsuki are the parents of Tazuna and genuinely care for him and would often tease him for having bonding moments with Koyori.
Koyori is the hand shaker partner of Tazuna, they are shown to be very trustworthy of each other and dearly want to protect each other. At first, Koyori reminded Tazuna of his younger sister Musubu but genuinely sees Koyori as her own person now. They are shown to be very loving of each other as well.