A second-year student at Sorami High, Hiyori Kazane becomes friends with Tomoki Sakurai and possesses strong feelings for him. Unknown to Tomoki, not everything is what it appears to be with Hiyori. She has a secret only Eishiro and the Angeloids Ikaros, Nymph, and Astraea know regarding her real identity.
Originally thought to be a shy, timid student at Sorami High School, there is much more to the klutzy Hiyori Kazane than most could ever dream about. She takes a unique, loving interest in the highly perverted Tomoki Sakurai, much to the surprise of everyone around her who are shocked that she can look over his perverted nature and see the caring side of him. Unknown to everyone (except the Angeloids), Hiyori Kazane is in fact an Angel from Synapse living out her "dream" in Sorami, and since it is usually a guarded secret of Synapse, they keep that information private from Tomoki and the others. It is when Hiyori is hit by a speeding truck and "dissolves away" right in front of Tomoki's eyes as she returns to Synapse that her true identity is revealed.
Hiyori Kazane was created by Suu Minazuki for use in the Sora no Otoshimono manga, and it is not fully known as to what inspired her creation. Hiyori first appears in Sora no Otoshimono: Volume Eight and is expected to appear in the upcoming film Heaven's Lost Property the Movie: The Angeloid of Clockwork.
When the shy and timid second-year student Hiyori Kazane first walked into The New World Discovery Club to ask permission from club leader Eishiro Sugata to join their club, it came as a shock to Tomoki Sakurai and all within the small club at Sorami High School. She was the first person other than those involved with their group that sought to join their club, and to Tomoki, this meant something suspicious was up. Not sure what to think of the sudden request, Eishiro is reluctant at first to accept her into the club, but since her joining their club will result in a larger budget from the school, he gladly accepts her into the New World Discovery Club. Hiyori Kazane is a mostly quiet, shy girl who joined the club in the hopes of getting closer to Tomoki (who she has strong feelings for, even though they have never been together before). Her kind behavior towards Tomoki (even when he acts extremely perverted around her) and disregard for his wild and twisted antics causes some emotional discomfort for not only Sohara Mitsuki (who secretly loves Tomoki), but for Angeloids Nymph and Astraea as well, for they too harbor strong feelings for Tomoki and do not appreciate this new girl getting involved.
Unknown to Tomoki, Sohara, and Mikako Satsukitane, Hiyori is not the simple 2nd-year student who she appears to be. In fact, she is hiding a major secret that if found out by others, would result in a complicated and potentially dangerous situation that could result in her death. In one of his earlier trips to the mysterious world of Synapse (using one of Nymph's machines to be transported there), Eishiro Sugata ventured into a massive domed complex located on one of the many floating islands in Synapse to explore the world of the angels in depth. After searching around, he came across a gigantic room filled with thousands of metal capsules that each contained the bodies of slumbering angels, each living out a "dream" on Earth as a way of keeping occupied in Synapse. While he was searching the room, he came across a capsule that contained a familiar figure, and to his horror and amazement, inside was a winged version of second year student Hiyori Kazane, sleeping peacefully within her capsule as she muttered something related to Tomoki. Only Eishiro (along with all three of the Angeloids back on Earth) knows the truth behind Hiyori's past and who/what she really is.
Tomoki, in shock that someone would be insane enough to actually join their club (which gets involved in wild, crazed misadventures on a regular basis), decides that action must be taken to prevent her from being with the club. Over the next few days, he tried everything that he could think up (a series of bizarre, perverted "purity" tests that he thought would prevent her from staying involved with the club) to scare her away to the point that she would never come back, but she completes all of his tests and did not leave the club (much to Tomoki's bewilderment). He begins to harbor feelings for Hiyori, who seems to always ignores his faults and treats him so kindly compared to Sohara and the Angeloids whenever he acts perverted in front of them.
He is flabbergasted and in complete disbelief when she comes forth and confesses her strong feelings of love for him (no one, especially a girl, has ever taken such a interest in him before, and this rocks Tomoki to the core). He begins to fall in love with Hiyori, and dreams of the things that they can do together in the future, but when he goes to confess his feelings for her, a major tragedy occurs. In a matter of seconds, she is brutally hit and killed upon impact by a slumbering truck driver who is traveling at a high rate of speed right in front of him. To his horror, she begins to dissolve away before his own eyes, and his memories of her existance begin to evaporate away (along with Sohara, Eishiro, and Mikako, who lose all memory of her existance after she dies).
Eishiro, who is currently making another trip to Synapse to learn more about the inhabitants who call it home, suddenly begins to forget everything he knew about Hiyori as she dies back on Earth. With only several minutes before every memory of her disappears forever, he charges past the twin Harpies (who are searching for him since he is trespassing there) and back into the dome, hoping to see her awake from her "dream" on Earth. To his amazement, Hiyori sits up from her sleep, a look of shock and horror on her face as she realizes that she has awakened from the dream and can never experience the same dream again. They lock eyes and seeing Eishiro before her brings tears to Hiyori, who knows that soon no one that she met back on Earth will ever remember her existence. She goes into a panic, and tries to re-dial the same dream, but is interrupted by the arrival of the mysterious Man of Synapse (along with the Harpies, his two Angeloid servants). He reminds her that she can never have the same exact dream again, and decides to let Eishiro go (since he has been entertained and Eishiro will soon forget everything in several moments).
Hiyori makes her return later in the story as a newly converted 2nd Generation Angeloid who is sent by her current master (who is The Man of Synapse) to eliminate Tomoki Sakurai, his fellow human friends, and wipe out Ikaros, Nymph, and Astraea. She was converted into a 2nd Gen. Angeloid (Type Zeta) by the Man of Synapse, who promised her that she could "continue" to live her life out on Earth. Wielding the powerful staff known as the "Demeter" (which grants her total control of the elements and weather around her), she creates a mighty vortex of wind that drops the air pressure down in the city of Sorami to dangerously low levels (to the point where Tomoki suffers crippling pain and begins to bleed from his ears as a result) in order to complete her primary objective. She is met by the three very enraged Angeloids (upset that Synapse would launch such an attack upon the poor inhabitants of Sorami) and engages them in combat.
It is only after Nymph uses her mastery of hacking abilities into Hiyori's system to override her system that frees Hiyori from the iron-fisted rule of The Man of Synapse and allows for her to return to Earth so that she can begin again. Tomoki is surprised when he finds out of her return and decides to become friends with her once again. But Hiyori decides shortly afterwords to resign from The New World Discovery Club and sets off to enjoy her new freedom on Earth as a free Angeloid, taking leave from being involved with their daily lives. She makes a rare appearance in later chapters of the manga, most notably when she discovers the traumatized and injured form of Chaos that is laying on the dirt road and is crying next to her vegetable garden after taking massive damage when attempting to return back to Synapse (which her former master, the Man of Synapse, forbids and launches a massive shot of pure energy via the powerful Zeus Canon at her, damaging one of her wings in the process). Hiyori is last seen watching in complete shock as Chaos launches up into the air and rushs off into the distance, making a beeline towards where Tomoki's house is. In the chapters that follow, Hiyori is seen from time to time, usually shown tending to her vegetable patch or doing minor chores.
Hiyori Kazane appears towards the end of the Ikaros Melan Arc, seen standing alongside Sohara and the others as Ikaros, Nymph, and Astraea struggle with the newest threat sent down from Synapse. Although she does not directly contribute towards the fight, she has come to Tomoki's house in her Angeloid gear in case that she is needed in the struggle against Ikaros Melan and the advanced Angeloids that the Man of Synapse has sent to kill Tomoki. In the aftermath of Ikaros Melan's defeat and the snapping of the Man of Synapse's sanity, Hiyori joins the others at the hospital to wish the battered Tomoki well as he tries to recover from his terrible injuries that he sustained at the hands of Ikaros Melan.
After being fully converted into an 2nd Generation Angeloid after the Man of Synapse gets his hands upon her to use against Tomoki and the other Angeloids down on Earth, Hiyori gains the powers of flight, speed, and the abilities to control and bend the elements around her to her will, made possible with the aid of her powerful Demeter staff. She uses these powers in order to carry out a failed attempt at killing Tomoki Sakurai by lowering the air pressure around Sorami to a dangerously low level, effectively putting the entire town at a high risk of death as from the lowered pressure around them, Tomoki begins bleeding from his ears.
Hiyori Kazane is scheduled to be one of the main characters in the recently-announced movie Sora no Otoshimono: The Angeloid of Clockwork when it is released later this year. The movie will take the arc that Hiyori appeared within the Sora no Otoshimono (Heaven's Lost Property) manga and portray those events within this feature-length film.
Heaven's Lost Property the Movie: The Angeloid of Clockwork
The first movie in the Heaven's Lost Property series, The Angeloid of Clockwork supposedly focuses on the Hiyori Arc from the Sora no Otoshimono manga.