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Voiced by: Haruka Shiraishi (Japanese), Xanthe Huynh (English)note
The main protagonist of the series who is a cheerful, energetic girl.
- Ambiguously Human: Aiko's body is made of artificial cells, making it both resilient to harm and a beacon for the Malignant Matter. Aiko is the first and only known case of an artificial organism with an artificial brain. Both she and the rest of the team wrestle with whether that makes her a human or something else.
- Amnesiac Hero: After losing her family in the Burst, she was mortally wounded and her brain was installed in an artificial body.
- Bridal Carry: Yuya gives her one in the first episode after she falls off her wheelchair.
- Clingy Costume: The suits that the Divers wear are made of artificial organisms, and attach to the body like a second skin. After Aiko puts one on, she has trouble getting it off again.
- Conveniently an Orphan: So she can go on adventures!
- Disappeared Dad: Aiko's father died in a car accident.
- Healing Factor: Shown in the first episode.
- Genki Girl: She's a cheerful, energetic girl.
- Ill Girl: She's in a wheelchair at the beginning. Until she Throws Off the Disability and is able to walk normally for the rest of the series.
- It's All My Fault: Aiko blames herself for pretty much anything that goes wrong during the journey, frequently exhorting her comrades to turn back so that they won't suffer further on her behalf.
- Late for School: It's how she ends up meeting Yuya.
- Made of Iron: Made of carbon nanotubes, actually. She can take being slashed across the face or falling several stories with just a little black bruising that quickly fades.
- Messianic Archetype: She sacrifices herself to destroy the Matter and free the real Aiko and her family from their Hell inside of it, and is then resurrected.
- Parental Abandonment: Both her parents died in the Burst.
- The Only One: Yuya in the first episode says that only she can stop the Burst.
- Throwing Off the Disability: When Yuya is introduced as the New Transfer Student, she's suddenly able to stand.
- Tomato in the Mirror: The Aiko that the series follows is actually the brain of the artificial Aiko (or "A.I.C.O.") in the real Aiko's body, while the Burst is the result of the artificial body reacting adversely to the real Aiko's brain.
- "Where Are They Now?" Epilogue: The original Aiko returns to having a normal life at school and home with her mother and brother.
A new student at Aiko's school and the other protagonist of the series with a mature and aloof demeanor.
- Aloof Dark-Haired Guy: He has a mature and aloof demeanor.
- The Atoner: He's responsible for the Burst and is now dedicated to ending it.
- Cassandra Truth: A strange one where he cracks the truth to Aiko, except only she doesn't believe it at first.
- New Transfer Student: To Aiko's school.
- The Stoic: He very rarely changes facial expressions.
- What Happened to the Mouse?: He disappears completely within the last minute of the series as Primary Point collapses. He's notably absent in the "Where Are They Now?" Epilogue, and whether he survived the collapse or not is never brought up, leaving his fate ambiguous.
Leader of the Divers. Shiraishi's partner.
- A Father to His Men: He always puts the needs of his team members first.
- Colour-Coded for Your Convenience: Red.
- Gentle Giant: Despite his tough and intimidating appearance, he is gentle and mild-mannered.
- Mentor Occupational Hazard: Shino is looked up to by all the other divers and is the only member of the group to die during the mission.
One of the Divers and Shinoyama's partner.
- Beauty Mark: Under her left eye.
- Colour-Coded for Your Convenience: White.
- Her Heart Will Go On: After Daisuke's death, Shiraishi struggles with grief, but ultimately pulls through.
- Innocent Blue Eyes
- Motherly Side Plait: She's not a mom, but she is a Team Mom.
- Nice Girl: She's very kind, gentle and easygoing.
- Rock Theme Naming: Her surname is spelled with the kanji 白石, which simply means "white stone".
- Support Party Member: She supports Shinoyama and the team with meticulous attention to everything ranging from vehicle operation and maintenance to making arrangements for food supplies.
- Team Mom: Maho Shirashi provides much of the team's tactical and emotional support.
- "Where Are They Now?" Epilogue: Shiraishi visits the site of Shino's death and leaves a bouquet whilst watching the reconstruction effort for the area.
One of the Divers and Kazuki's partner.
Voiced by: Taishi Murata (Japanese), Khoi Dao (English)note
One of the Divers and Sagami's partner.
- An Arm and a Leg: Kazuki lost his right arm in a Matter attack, and had it replaced with a prosthetic. Over the course of the series, he has to replace the arm a few times.
- Colour-Coded for Your Convenience: Green.
- Floral Theme Naming: Kazuki's name is spelled with the kanji 一樹, which simply means "a tree".
- The Dulcinea Effect: Kazuki is quick to fall for Aiko and even help during her initial escape attempt from the group without even knowing what was going on.
- The Smart Guy: He uses his talents in electronics to support his partner Sagami.
- "Where Are They Now?" Epilogue: Kazuki is working at centre for those with prosthetic limbs.
One of the Divers and Kaede's partner.
- Aloof Dark-Haired Girl: She's cool-headed, beautiful and intelligent.
- Color Motif: Purple.
- Colour-Coded for Your Convenience: Purple.
- Floral Theme Naming: Haruka's surname is spelled with this kanji 芹, which means "Java waterdropwort" which is a more common name for Oenanthe javanica which is a kind of plant.
- Graceful Ladies Like Purple
- Hot Scientist: Before the Burst, she was an artificial life form researcher.
- Smoke Out: In episode 3, she uses a smoke grenade to help her escape. Purple smoke too!
- "Where Are They Now?" Epilogue: Haruka goes back to work at the research lab with Kurose.
One of the Divers and Haruka's partner.
- Close-Range Combatant: She specializes in close quarters combat and preemptive strikes.
- Colour-Coded for Your Convenience: Orange.
- Floral Theme Naming: Kaede's name is spelled with this kanji 楓, which means "maple (tree)".
- Girlish Pigtails: Has two very small ones.
- Significant Green-Eyed Redhead
- "Where Are They Now?" Epilogue: Kaede is working as a kindergarten teacher.
One of the artificial life form researchers at the Kiryu Biotech Research Facility.
Head of Kiryu Hospital and the doctor in charge of Aiko's care.
- Anti-Villain: His sole motivation is to restore his daughter from her coma. Unfortunately, he doesn't realize that he's the one keeping her there, and by the time Yuya tells him, he's too far gone to listen.
- Healthcare Motivation: He does what he does so he can get his daughter out of a coma.
- Labcoat of Science and Medicine: Because he's a doctor.
- Villainous Breakdown: Goes insane after linking to Yuzuha's clones.
Chief of the CAAC (Control Agency of Artificial Creatures) Response Bureau and a central figure in the government's efforts to promote artificial life form technology.