Hiyori is introduced as a second-year student at Tomoki’s school. She has feelings for Tomoki and joins the New Continent Discovery Club just to get closer to him.
In actuality, Hiyori is an angel who was first seen when Sugata went to Synapse. She was inside some sort of a sleeping chamber that connected her to her body on Earth, which functions like an avatar.
Sugata was hesitant on letting her join the New Continent Discovery Club, since there are still a lot of things he does not know about what she really is. However, he welcomes her anyway, since her membership will allow his club to receive more funding. Thus far, nobody, except Sugata, is aware of Hiyori’s true identity.
When Hiyori’s body in Synapse awakened, the Hiyori on Earth “died” and everybody forgot her. In Synapse, she was turned into a Weather Weapon Angeloid called Zeta, and was ordered to destroy the humans by decreasing the air pressure. Ikaros, Nymph and Astraea tried to fight against her, but they couldn´t because she was their friend. Nymph tried to hack but Zeta have a much more powerful ability to counter-hack Nymph. Ikaros, Astrea and Nymph team up together to destroy the machine that is latched onto Zeta’s back, thinking that it will stop her. After the machine is destroyed, Zeta’s necklace start a countdown, a time warping bomb. Ikaros used Aegis around Zeta and she also when into it, saying that she doesn’t want Zeta to die alone. Tomoki intercepted saying he would not want both to die. Zeta was woken up by memories when Tomoki talked to her. She knew that she was going to die. She hacked Ikaros and told her to bring Tomoki to safety. Zeta imploded to Tomoki’s despair, despite him promising that he would save her. She also knows who Tomoki likes (and it isn’t her). At the ending of the movie, they show that Zeta and Tomoki have actually met eachother when they were young. After the credits of the movie, Tomoki is shown standing on the aforementioned hill, when suddenly feathers fall all around him, as he turns around he starts smiling. After the screen fades out, the sound of Hiyori’s bells is heard. Hinting that she is still alive, just like in the manga where the implosion (from the movie) never happened and was prevented by Nymph. After which Hiyori comes back to school as an Angeloid.