Mesus Island resident and an amateur pilot. After witnessing his family's death in a Sky Clan attack, he is driven with a resolve to join the military, become the Sky King, and crush the Sky Clan for the world to usher in an age of peace. At the beginning of Seiyaku, Kiyoaki is a third-year student at the Kanan Military Academy in Akitsu. His high grades allow him to take up a transfer spot at Eahanto to finish his studies.
- Ace Pilot: Is reported to have over 300 kills by volume 9.
- Balcony Speech: Kiyoaki gives a bombastic speech to the allied forces' pilots in volume 9 to raise their battle morale.
- Battle Couple: With Ilya in volumes 4 and 7-9.
- Bridal Carry: Once they are ready to leave Pleiades at the end of volume 9, Kiyoaki has Ilya assume this posture inside his fighter cockpit due to a lack of space.
- Call to Agriculture: After resigning from his instructor pilot position during the epilogue, he uses the royalties from his memoires to buy back his family vineyard on Mesus.
- The Determinator:
- Doomed Hometown: The death of Kiyoaki's family and the bombing of his hometown in the prologue drives him to enter the air force.
- Emotion Suppression: To cope with the stress of aerial combat and killing people in volumes 4 onward, Kiyoaki coaches himself to "become" a lump of steel in battle.
- Famous Ancestor: His father Masaharu was a famous pilot during the last Akitsu-Saint Vault War, and was known for shooting down Karsten Kleischmidt in a one-on-one air duel.
- Heroes Want Redheads: After Mio formally defects to Uranus at the end of volume 3, Kiyoaki's relationship with Ilya deepens.
- Heroic Second Wind: During the air duel with Carnation in volume 9, Kiyoaki begins performing Pugachev's Cobra and suffers through the torment of the high Gs. The auditory hallucinations of his friends praising his skill gets him through the maneuver and shoot down Carnation.
- Marshmallow Hell: An unwilling recipient of one from Kagura. She hunts him down for a Cooldown Hug after losing a mock air battle in volume 2.
- Living Lie Detector: In volume 2, he asks Raina about his past, in which his intuition detects as lies when he answers.
- Piggyback Cute: He gives Ilya a couple of these while she's drunk in volume 4 during the Vortex Squadron's evening parties.
- Returning the Wedding Ring: Kiyoaki tossing the childhood wedding ring in the flames during his meeting with Mio in the Pleiades' palace. It symbolizes that they can't be with each other anymore and that both have mutually acknowledged moving on from what they thought of each other three years ago.
- Revenge: His main motivation for deciding to join the military.
- Love Triangle: Kiyoaki invokes this in his mind when thinking about Mio and Ilya while staring at the night sky. It makes him feel like shit for having to choose.
- Thou Shalt Not Kill: His moral code in volume 2, which is a hindrance to his performance in mock air battles. His later Emotion Suppression gets him over this.
- Wham Line: He gets one from Mio at the end of volume 3, when she tells him about her status as a spy, and to forget about her.
Mesus Island resident and Kiyoaki's Childhood Friend. She has had quite the transient childhood as her adoptive father is a civil servant in Saint Vault's foreign ministry. Mio has known Kiyoaki since they were ten and enters officer school with him after the Sky Clan's attack on Mesus.
- A Match Made in Stockholm: Mio with Raina after several volumes and loads of character development on Raina's part. At the end of the story, Mio decides to act on her current circumstances rather than stick to the past.
- Break the Cutie: Her experience from the second half of volume 2 to the end of volume 3.
- Cannot Spit It Out: The cause of her no-contact policy with the other characters in volumes 2-3 is her reluctant choice to be a spy. Even with the other characters pressing her for the reasons of her troubles, she doesn't dare expose her role lest her family be endangered.
- Childhood Friend Love Interest: Played straight in the first half of volume 2 when it's obvious she and Kiyoaki are developing feelings for each other beyond just being old friends. Her unwilling defection to Uranus gives this trope a more dramatic flavor. When the duo meet each other again in volume 9, they both acknowledge moving on from this phase.
- Clingy Jealous Girl: Towards Kiyoaki after the "tripping incident" in volume 1.
- Childhood Marriage Promise: Kiyoaki and Mio made one when they were 12, where Kiyoaki put on a flower tiara and ring on her on a summer day. Events from volume 2 onward make this impossible, although both reference it in their internal monologues.
- Dude Magnet: Mio gets some unwanted attention in attending a banquet with Balthazar in volume 2 and when accompanying Nina Viento at a public swimming pool in volume 5. Her physical attractiveness is why Xeno is so intent on training her as a spy.
- Femme Fatale Spy: Xeno sees potential for developing Mio into one.
- Forced into Evil: Mio agrees to become a Uranus spy because Xeno threatens to induct her siblings as substitutes if she doesn't cooperate.
- Green-Eyed Epiphany: In volume 2, Mio witnesses Kiyoaki and Ilya returning from a sightseeing trip where the duo part on friendly terms. She experiences extreme jealousy and sorrow, and also feeling unworthy in comparison to Ilya due to her spy status.
- Hair of Gold, Heart of Gold: This and her Innocent Blue Eyes are visual signs of Mio as a morally good character. It makes her spy status much more impactful.
- Happily Adopted: After losing her birth parents at the age of 8, Mio is adopted by Ethan Seira.
- Heroic BSoD: She has one in volume 3 after learning that her adoptive father leads an international arms dealing conspiracy to foment wars.
- Insecure Love Interest: After witnessing Kiyoaki and Ilya say goodbye together in vol 2, and after Raina tells her that Kiyoaki and Ilya embraced and were reluctant to let go in vol 6.
- Intimate Healing: Kiyoaki and Mio are shot down during a naval training flight in volume 3. After crash-landing on the shore, Kiyoaki suffers from a head wound and hypothermia and is likely to die without immediate treatment. Seeing no other option, Mio has to undress him and herself to share body heat.
- Nothing Is the Same Anymore: Mio says this to Raina when they witness the big attack in volume 3
- Lady-in-Waiting: Mio becomes one to Claire during the Succession Crisis and afterwards.
- Massive Numbered Siblings: Her family has *eight* siblings. We later learn that they are adoptees from war zones.
- Meet Cute: The nature of her father's work has meant Mio never really established any friendships. She then made bird Fio to find a friend for her, leading to Fio landing on Kiyoaki. She subsequently declared him as her "servant", which is how their friendship began.
- Necessarily Evil: By being a spy, Mio chooses the safety of her family over the trust of her friends.
- Not Himself: Mio returns to the academy after winter break and Kiyoaki notices that there's something amiss about her. She no longer wants to talk to people and avoids all the other six characters for months. Volume 3 reveals that she has been forced to become a spy for the Sky Clan, and feels guilty and extremely conflicted over her circumstances.
- Pity Sex: During Intimate Healing with Kiyoaki in volume 3, Mio does this while he's in a delirious state of mind. Despite both denying it, the act only makes her feel more guilty and unworthy of a friend due to her father's responsibility for the attack on Mesus.
- Regretful Traitor: While she agrees to become a spy
- Shameful Strip: Mio relucantly complies with Xeno's order to completely strip in front of him in volume 7.
- Team Chef: The rest of the Eriador Seven love her pasta and katsudon at dinner time.
- Tears of Fear: At the end of volume 2 when Raina asks her a coded question to check if she's a fellow spy.
- Thicker Than Water: Her reasoning on choosing to be a spy and betraying her friends. If she declines, then Xeno will make her siblings engage in espionage.
- Took a Level in Badass: After relentless practice in infiltration and combat skills, she becomes an expert marksman and breaks all of Xeno's joints in hand-to-hand combat in volume 9.
- Troubled Fetal Position: This is her coping mechanism in volume 3 for becoming a spy.
Third-year student at Eahanto and the daughter of Karsten Kleischmidt. Thanks to her upbringing, she is totally devoted to the art of aerial combat and aspires to be a worthy successor to her father. Her cold and blunt personality has made her few friends. She also holds Kiyoaki in contempt, believing that his dad shot down Karsten using dishonorable tactics.
- Abusive Parents: Karsten has been brutal to her in the past. He was said to curse that Ilya was born a girl rather than a boy, and his drunken outbursts drove her mother out of the house.
- Ace Pilot: By volume 6, she's probably the highest-scoring ace in the Saint Vault Air Force.
- Attempted Rape: In volume 1, Ilya wanders in a forest alone and is confronted by natives wanting to assault her. Kiyoaki intervenes armed with a pistol but is ambushed by another native. Kagura arrives in time to save them both and sending the other natives scrambling for safety.
- Alcohol-Induced Idiocy: After getting madly drunk at a party in volume 8, Ilya dons a revealing swimsuit and dances in front of an ogling audience...until Kiyoaki pulls her out of the scene while flustered that Ilya would do such a thing.
- Boyish Short Hair: She had a masculine hairstyle before attending the military academy. After meeting Cecile, she switched over to a more feminine style shown in her portrait.
- Closet Geek: Cecile tells Kiyoaki that someone in the academy anonymously makes donuts and places them in the cafeteria for everyone to eat. Ilya is confirmed to be their maker at the end of volume 2 and gets flustered when Kiyoaki says that he enjoys them. A foreshadowing hint appears earlier when Ilya stops by at a confectionary store that she frequented as a child.
- Defecting for Love: The end of volume 6 has Ilya and Kiyoaki colliding into each other in battle before bailing out and landing on the same spot. After being picked up by Achmed, the duo desert and join his Valkyrie Squadron, knowing that returning back to their units means that they'll be separated again and will fight each other for the rest of the war.
- Defrosting Ice Queen: Ilya barely tolerates Kiyoaki as a co-pilot of Eriador in volume 1. From volume 2, she doesn't only grow to respect and like him, but also sheds much of her emotionless behavior and builds up camaraderie with her pilot peers.
- Emotionless Girl: A symptom of her Ice Queen state of mind. It is shown most prominently in volume 2 where she suppresses all her emotions while flying mock battles.
- Famous Ancestor: She is the child of ace pilot Karsten Kleischmidt.
- Hard-Drinking Party Girl: Revealed to be this when she is peer-pressured to drink in volume 4. Ironically she had a personal abstinence from alcohol due to seeing what it did to her father.
- High-Pressure Emotion: Ilya has this reaction in volume 9 while listening to Kiyoaki's proposal to her during the evening before the final battle, but this reaction is described more in terms of her brain nerves "burning away" rather than steam.
- Hysterical Woman: Kiyoaki's proposal is too much for Ilya to take in, so she reacts by slapping and verbally abusing him, and then crying and babbling nonsensically for the rest of the chapter.
- Important Haircut: Ilya gets one at the beginning of volume 7. Upon joining the Valkyrie Squadron, her status as a Saint Vault pilot would make her presence difficult, given that she is technically deserting and violating military law. She cuts her hair, changes its color, and adopts a nom de guerre. The haircut also represents Ilya shedding away at her old personality established in the early narrative.
- Insecure Love Interest: Ilya experiences a phase of this in volume 9 when she ponders about the implications of Kiyoaki rescuing Mio from Pleiades. It is soon rendered redundant when Kiyoaki proposes to her moments later.
- Monument of Humiliation and Defeat: Carnation shoots down Ilya at the end of volume 3. Ilya bails out but Carnation mocks her by flying circles while she descends to the ground.
- Nightmare Sequence: She begins having one in volume 1 where she is involved in an air battle over a burning city while shooting Kiyoaki's plane out of the sky. She dismisses it as a wild delusion, but it becomes more painful to experience after she develops romantic feelings for him in the later volumes. It foreshadows the climax of volume 6.
- Second Love: Eventually to Kiyoaki after the events of volume 3.
- Teeth-Clenched Teamwork: Her with Kiyoaki in volume 1, when they are responsible for flying Eriador to Saint Vault.
- Tempting Fate: Invoked. The day after freaking out from Kiyoaki's marriage proposal, Ilya ambushes him back with her own betrothal demand, playfully explaining that she wants to jinx him.
- Tomboy with a Girly Streak: Volume 2 describes Ilya as a tomboy in hairstyle, conduct and speech when she was younger. We later know that Ilya secretly loves making confectioneries with cartoon animals as decorations.
- Unbroken Vigil: Ilya takes watch over Kiyoaki in the hospital after an aviation accident in volume 2.
Third-year student at Eahanto. He is described having a carefree personality and being a Handsome Lech. He is actually a teenage spy codenamed "Hummingbird", responsible for collecting intelligence for the Sky Clan's military commanders to plan out surprise attacks on Saint Vault.
- Armor-Piercing Slap: Upon rushing over and seeing that Kiyoaki tripped over a naked Ilya on the beach in volume 1, Raina sarcastically praises his boldness and says that he underestimated him. Mio slaps Raina in response.
- Consummate Liar:
- Embarrassing Nickname: Upon returning to Pleiades after being compromised by Kiyoaki, Xeno becomes disappointed at Hummingbird's performance and calls him "chinkasu" or "dick cheese".
- Even Evil Has Loved Ones: His motivation for joining Uranus' intelligence service is to pay for his sickly mother's medical bill.
- Hidden in Plain Sight: Raina sends intelligence reports back to his superiors by giving "love letters" to a courier operative.
- The Mole: For Uranus under Xeno's orders.
- Revenge: He is the son of a very high ranking Sky Clan politician who was arrested for corruption and executed. He wants to advance through the intelligence service's ranks so he can get back at those who accused his dad.]]
- Secret Identity: "Raina" is one of his disguises which he can switch in and out willingly.
- Split Personality: Raina is actually "Hummingbird" and can switch between personalities freely. He also has a third personality Thomas (his true personality)
- Two Aliases, One Character: "Hummingbird" gets his own narrative in the story. By volume 2 the reader learns that Raina and "Hummingbird" are the same person.
Second-year student at Eahanto. She's the youngest of the Eriador Seven and constantly keeps an optimistic attitude. Her real name is Elizabeth Cecile Sylvania, the heir to the annexed Sylvanian kingdom.
- Communications Officer
- Dawn of an Era: Gives a radio address outlining her goals as the leader of the restored monarchy, and her policy goals towards the Sky Clan.
- Genki Girl: The most optimistic member of the Eriador Seven.
- King Incognito: For all intents and purposes Cecile is an ordinary teenage girl with no distinguishing hobbies or conduct that mark her as royalty.
- Noble Fugitive
- Really Royalty Reveal: In volume 5, when Balthazar deduces that she is actually Elizabeth.
- Rightful King Returns: By volume 7, Cecile is coronated as Queen Elizabeth of the restored Silvania monarchy.
- Secret Identity:
- Royal Blood:
Fourth-year student at Eahanto. Like Kiyoaki and Mio, she hails from Akitsu but has been a transfer student for a longer period of time. In volume 1 she serves as the second-in-command of Eriador.
- Hime Cut: As shown in her portrait.
- Kill the Ones You Love: During the coup in volume 8, she takes a hand in killing a number of anti-coup soldiers and her own brother—a commander in the loyalist guard force.
- Lady of War: She's skilled in both flying a fighter and using a sword.
- Legacy of Service: Kagura's family has served Akitsu's ruling dynasty for generations. Once she's back to Akitsu she is assigned to the royal guards rather than the military.
- Reasonable Authority Figure: At the end of volume 1, Balthazar rejects Kiyoaki's request to land the flying boat immediately, explaining that they could crash in the rough seas. Kagura suggests a vote amongst the crew
- Reports of My Death Were Greatly Exaggerated: When Balthazar visits Kagura's gravesite, she appears out of the void. Kagura explains that her execution was recorded in the newspapers, but was secretly pardoned by the new emperor and was only shot in the right shoulder. However, she can no longer wield a sword.
- Replacement Goldfish: When Kiyoaki meets Kagura for the first time, he is quite startled that she looks like his deceased sister Yumiko. This resemblance drives his personal outlook towards Kagura.
- Shot at Dawn: Even though the Akitsu coup succeeds, Kagura takes responsibility for spilling blood on sacred palace grounds and killing a number of fellow soldiers. She is reported to the press as executed at the end of volume 8.
Fourth-year student at Eathanto. An analytical and stoic person, he is the overall commander of Eriador in volume 1, for better or worse. Balthazar aspires to get on the fast-track of officer advancement and land a senior position in the military's general staff.
- Bad Boss: Balthazar's leadership style in volume 1 is stifling. He tells Kiyoaki not to question orders when asked to justify them, and he prefers to have Mio bleed to death rather than to risk the entire crew in an emergency landing. He never assumes a leadership position again after graduation, instead being assigned to intelligence analysis and staff work.
- Consummate Professional:
- Ensign Newbie: In volume 1.
- Glory Hound: From volume 2 onwards, it's quite obvious that he wants to "get known" and rise through the ranks unnaturally fast.
- Military Maverick: Balthazar is assigned to participate in staff wargames after his assignment as a low-level intelligence analyst. His unconventional plans and courses of action earn in many tabletop victories to the displeasure of his old guard superiors.
- Oblivious to Love: His attitude towards his career and personal motives has made him to reject Kagura's advances. This stance gradually erodes over the series and after the Uranus war he decides to act on his feelings and marry Kagura.
- Overranked Soldier: From officer cadet to brigadier general over the span of four years.
- The Runaway: Balthazar running away from his zaibatsu family.
Saint Vault Empire
An officer in the Saint Vault Imperial military, and commander of the elite Vortex Squadron.
Kingdom of Silvania
Commander of the Valkyrie Squadron, Silvania's Ace Pilot, and Kiyoaki's mentor.
Bodyguard of Claire Cruz/Nina Viento. Was a prominent character in Toaru Hikushi e no Koiuta.
Queen of the Sky Clan. Was a main character in Toaru Hikushi e no Koiuta.