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.
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.
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 isKaran. 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.
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.
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.
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.
Ambiguous Disorder: Often seems lost in social situations, such as her proposal to Shion or cluelessness about romance when studying abroad. However, her behavior may just be a result of growing up as a sheltered child prodigy.
Damsel in Distress: After being captured by the Bureau of Public Security. Turns into a Badass Damsel when she uses Elyurias' power to help Nezumi and Shion by controlling the Correction Facility's computers, although her role consists less of kicking ass and more of standing around and looking kinda creepy.
Heroic Sacrifice: When Nezumi and Shion blow up Mother, she stays behind with Elyurias in order to destroy it for good.
No Social Skills: She functions well in No. 6, however she's seen as odd by her peers when she studies in No. 5. It certainly doesn't help that her request to Shion before she leaves to study abroad is "I want to have your sperm."
Unlucky Childhood Friend: Shion flat-out states in the novels that while he treasures her as a friend, he does not and cannot return her romantic feelings. Aside from Shion's attachment to Nezumi, circumstances just do not work in her favor.
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.
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.
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.