Her "best friend." Yoshino is very attached to her puppet, Yoshinon believes itself to be real and will get extremely (and dangerously) emotional when Shido insists that she is doing ventriloquism (for the most part, she seems unaware of it) or when something happens to it. Yoshinon is "the person that I [she] want[s] to be" and it is her "hero." As a Spirit, Yoshino was dependent on Yoshinon to the point that it become her mental support. Without Yoshinon, she would not only cry a lot, but also freeze everything in the area.
Yoshino’s new hero, savior, and the next person she depends on besides Yoshinon. She respects him a lot and looks up to him as a little sister would look up to an older brother. Yoshino trusts him greatly and is grateful for him in finding Yoshinon. This trust is what allowed Shido to seal her powers. However, Yoshinon's behavior hints that her feelings may be deeper.
At first, Natsumi simply used Yoshino's puppet, Yoshinon, as her disguise during her 'game'.
However, after Natsumi's spirit mana was sealed, their compatible and similar personalities allowed them to become good friends with each other.