A boy raised among the elite of No. 6, only to be stripped of his privileges after giving shelter to Nezumi, a fugitive. He is unfailingly polite and idealistic and refuses to give up on saving the people of No. 6.
- Adorkable: Often.
- Adaptation Dye-Job: His eyes were violet in the novels as opposed to the red in the anime and manga.
- Back from the Dead
- Badass Adorable: He seems all cute and innocent on the outside, but mess with someone he cares about, specifically Nezumi, get ready for the beatdown of your life.
- Beware the Nice Ones
- Bishōnen: Not nearly on the same level as Nezumi, but he still qualifies.
- Brainy Brunette: Before the Wasp, anyway.
- Break the Cutie: Shion, in the Correctional Facility
- Child Prodigy
- Did Not Get The Guy Or Girl: Not only does Nezumi leave him behind when they're done, but Safu, his Het Option, Ascends to a Higher Plane of Existence. However, there might be hope because Shion promises that he'll meet Nezumi again at the end of the anime and novels.
- Disappeared Dad: Mentioned only in the novels. His mother claims he was careless with money and women and was borderline alcoholic.
- Beyond gives more information. Also, Nezumi meets him.
- Disease Bleach: The reason for his white hair.
- Even the Guys Want Him: Nezumi, of course, but he was also hit on by a greasy old corpse-collector, with promises of drinks and meat, no less.
- Friend to All Living Things: He took in the fugitive Nezumi and treated his wound when they were children, dooming his bright future in the process. Years later, Nezumi's mice take a liking to him (he even gives them names), he reads to the neighbouring kids in the Western Block, he gets along with Dogkeeper and his/her dogs, and Rikiga even calls him an "angel" at one point.
- And in the end, he adopts the ridiculously cute baby he saved two episodes earlier.
- Genius Ditz : Even though Shion is naive and lacks a sense of self-preservation, he was ranked the highest in the city's intelligence test when he was just two years old.
- Ho Yay: With Nezumi. Invoked deliberately, and by the end, confirmed.
- Locked into Strangeness: After his encounter with the wasp. Shion after surviving the wasp being born. He also gains a red snake-like mark curled around his body from his ankle to his neck. Not that Nezumi minds.
- Love Makes You Crazy: When Nezumi is threatened, Shion switches from either all-out rage or cold, completely rational and murderous.
- Momma's Boy: Well his mom is Karan. Shion was raised solely by his mother and both of them are very (happily) devoted to one another.
- Morality Pet: For Nezumi. An unusual example in that he struggles with moral issues himself— in fact, he becomes more willing to hurt others through his association with Nezumi, in a sort of reverse Morality Pet effect.
- Meaningful Name: "Shion" is a flower (specifically, a type of aster) with medicinal value. The flower also means "I will remember you" or simply "remembrance" in Japanese tradition, which could allude to the ending.
- Nice Guy
- The Pollyanna
- Red Eyes, Take Warning: Averted most of the time. Threaten Nezumi, however, and see what happens.
- Spell My Name with an "S": Shion/Sion.
- Violently Protective Boyfriend: Do NOT insult or endanger Nezumi.
- Wide-Eyed Idealist: He's more on the cynical side, but firmly believes that a bright future is attainable.
- In the novels, Shion is close to strangling a man because he tried to kill Nezumi. Only when Nezumi tells him to stop he snaps out of it. And then there's his explanation of what he was doing:
- That man tried to kill you, so he had to receive punishment.
- He also violently attacks Rikiga when the older man makes a pass at Nezumi and later kills a guard point-blank in the head after he shoots and injures Nezumi.
A cynical young man with a dark past. He was saved by Shion when they were younger, only to return the favor years later
. Due to his experiences in No. 6, he hates most everything about the city and its people, and is perfectly willing to watch it fall.
- A Boy and His X: With his mice.
- Attractive Bent-Gender: As his local theatre was swift to notice, Nezumi makes a really good-looking woman.
- Badass: He's awesome personified in this world.
- Bishōnen: Pretty enough to be hit on multiple times and to play in drag as the beautiful Ophelia.
- Cultured Badass: He knows a lot about literature and is an actor by trade.
- The Cynic
- Dark and Troubled Past
- Dude Looks Like a Lady: He's a talented actor and performs in Shakespearean plays for a living. Shion first sees him playing Ophelia in Hamlet.
- Easily Forgiven: Shion was furious when Nezumi admitted that he was using Safu as an excuse to break in the facility for his own agenda. All is forgiven when Nezumi is seriously injured (only a few minutes after the revelation) and Shion rushes to heal him.
- Even the Guys Want Him: Shion being the main example. There is also the two mooks who wanted "payment" from him after he kisses the prostitute, and is also implied by Rikiga when he offers Nezumi a job as a gigolo.
- Gray Eyes: Occasionally silver.
- Ho Yay: With Shion. Invoked deliberately, and by the end, confirmed.
- Jerk with a Heart of Gold: He can be a jerkass at the very least. But he also sings to Dogkeeper's dying pack members to ease their suffering, to say nothing of what he'll do where Shion is concerned.
- Only Known by Their Nickname
- Last of His Kind: Nezumi is all that is left of the Forest People that lived on the site where No. 6 was built.
- Tall, Dark and Snarky: Oh yes.
- Magical Native American: He can "hear" nature, and can sing a song of calming.
- Meaningful Nickname: "Mouse" or "rat", a name given to him after escaping the destruction of his homeland and the slaughter of his people. His real name is never revealed in-series.
- He also goes by "Eve" when onstage. Two guesses as to who Adam is...
- Real Men Wear Pink: He's extremely tough and stoic, but also loves reading and singing, and regularly performs in drag.
- Scars Are Forever: The massive burn scars on his back are courtesy of the flamethrower-wielding No. 6 soldiers that slaughtered his people.
- Tsundere: Especially towards Shion. He'll deliver an angry rant at No. 6, but the moment he sees Shion is upset he gently comforts him.
- What Beautiful Eyes: In the novel, Shion almost cannot mention Nezumi without talking about how beautiful his eyes are.
- Wholesome Crossdresser: As part of his acting job. He's very good at it, too.
- You Gotta Have Blue Hair: Blueish, grayish, off-purpleish, depending on your point of view.
Shion's childhood friend and a fellow resident of No. 6. Like Shion, she's a child genius - her expertise is the human brain and to a degree, psychology. Also like Shion, Safu begins to show doubt about the utopia that is No. 6.
One of the residents of the West Block outside of No. 6. Inukashi, quite literally "Dog Keeper," is a brash, often dangerous individual who spends his (or her) time either renting out his many dogs as heaters during the winter or collecting items to sell from an illicit contact within No. 6's correctional facility. He has a very violent, volatile relationship with Nezumi, but Nezumi still often goes to him to gather information - for a price.
- Ambiguous Gender: Assumed by some to be female, based on his small stature, nice legs, and one mention of 'softness' when he was hugged, however official information does not give a definite answer one way or another.
- Samus is a Girl: Treated by the other characters as male, until used as bait to lure in a No. 6 higher-up: Shion, trying to comfort Inukashi afterward, wonders if his friend might actually be a girl. The rescued baby from episode 10 also calls Inukashi "mama". Even so, Inukashi never personally discusses his gender identity, so there's no way to know.
- Crossdressing Voices: Sort of.
- Berserk Button: Inukashi does not react well when his dogs, especially his 'mother', are insulted.
- Meaningful Name / Only Known by Their Nickname: Inukashi literally means Dog-keeper, or Dog-loaner.
- Raised By Dogs: His "mother" was a dog that offered milk and kept him safe.
- Tsundere: Only towards Shion.
Shion's mother. With Shion, she was sent to live down in Lost Town where she opened up a bakery. She and Nezumi contact one another through strips of paper brought by mice.
- Dude Magnet: She has a lot of admirers...
- Friend to All Children: Thinks children are adorable, and strongly believes it is an adult's responsibility to protect them.
- Good Parent: She really loves Shion and isn't upset at all about their downgrade from a life of luxury.
- Mysterious Past: At least one photo of her exists with the Founding Members of No.6. She also knew where Rikiga was likely to be in the West Block.
One of the residents of West Block. He runs a prostitution service which allows him a life of luxury in comparison to other residents. At one point, he wrote articles regarding No. 6.
- Cowardly Lion: He really doesn't want to get involved in Shion and Nezumi's plans, but does so anyway.
- Fanboy: He really likes Eve, Nezumi's stage persona. He's less charitable towards Nezumi, given that well, it's Nezumi.
- Intrepid Reporter: Hinted that the reason he was sent to live in West Block was due to his articles questioning No. 6's activities.
- Parental Substitute: He tries to be this to Shion from time to time, given that he's still attached to Karan.
One of the residents of No. 6, and a resistance fighter. Yoming first begins to suspect something is amiss in the city when his wife and child mysteriously disappear. He takes interest in Karan after Shion's disappearance.
- Conspiracy Theorist: Luckily (or unluckily), the conspiracy is real.
- Karmic Death: Though starting out with the intention of avenging his family's death, he eventually descends into creepy and unwanted advances on Karan and gleefully planning to incite a mob bloodbath on No. 6's founding anniversary. He ends as another victim of the birth of the wasp.
A mysterious entity, subject to much research by the scientists of No. 6. Elyurias once existed in the forest that was destroyed by No.6's operatives, and has power over nature. She was worshiped by the Forest People as a god. No. 6's government has gone to great lengths to research how to control her power.