The AI replicating Chiaki Nanami is a participant of the Killing School Trip. Her title is the Ultimate Gamer.

During the events of The Biggest, Most Awful, Most Tragic Event in Human History, a psycho-therapeutic virtual reality rehabilitation program known as the Neo World Program or the Hope Restoration Program was activated by Makoto Naegi, a member of the 14th Division of Future Foundation, to undo the trauma and brainwashing inflicted on the Remnants of Despair. They were former members of Hope's Peak Academy's Class 77-B who had been turned into tools of despair by the mastermind of the Tragedy, Junko Enoshima.

Using the research conducted by the Ultimate Programmer Chihiro Fujisaki, a co-creator of the Neo World Program, two non-playable characters known as the Observers were created. Their task was to help the overseer Alter Ego, Chihiro's artificial intelligence program, to directly guide the 15 participants and carry out the pre-programmed objectives of the session. The first Observer was Usami, who assumed the teacher role and had the authority to create and manage rules.

The second Observer was to fill the student role, thus not having any special authority to blend with the participants. They were initially programmed to take the appearance and personality influenced by the thoughts and memories of the users, such as esteemed authority figures, and fuse their personalities into one AI. However, the collective conscience of all participants, including one Reserve Course student Hajime Hinata, desired to see their beloved friend, Chiaki Nanami. She was the late Class 77-B representative whose death had been exploited by Junko to brainwash them into despair. As a result, the program was altered to replicate Chiaki's appearance and personality, identifying herself as the Ultimate Gamer.

Due to the nature of her creation, the AI Chiaki considered Chihiro as her father, and Alter Ego and Usami/Monomi as her siblings. Though she was fond of her father, the knowledge she had about him was mostly hearsay, as he had passed away prior to the Neo World Program activation. She inherited Chihiro's ineptitude in the dating sims genre or socializing in general, as well as his programming skills and kindness. She was also programmed never to reveal her identity as an Observer on her own, or later during the killing game, as "the traitor."

The Student Observer, now assuming the identity of Chiaki Nanami, joined the ranks of the Remnants of Despair once the Neo World Program commenced. With everyone's school memories wiped out as part of the proceedings, Chiaki slotted in perfectly into the group, acting as their classmate.

Chiaki was focusing on her game when Hajime first met her. She ignored them until Nagito Komaeda, the Ultimate Lucky Student, called out to her. She would take lengthy pauses or not bother talking unless prompted while introducing herself to Hajime. She said that she had to think everything she had to say before articulating it, before becoming sleepy and tired, cutting the introduction short.

Once gathered at the first island beach by Usami, Chiaki reminded everyone that they could leave the island sooner if they continued collecting Hope Fragments. Despite this, she didn't join them playing at the beach, thinking she would drown if she went to swim with them.

Though the familiarization sessions with the virtual reality had been proceeding smoothly, a virus secretly implanted by Izuru Kamukura, the Ultimate Hope, and Hajime's identity during his time as a member of Ultimate Despair, suddenly activated, allowing Alter Ego Junko, manifesting herself initially as Monokuma, to hijack the simulation. Chiaki, who had been calm and unfazed so far, became concerned and rushed to Jabberwock Park with everyone else, where Monokuma had been waiting.

Once Monokuma announced his plan to turn the harmless simulation into the Killing School Trip, Chiaki helplessly witnessed Usami being stripped of her power and forcibly renamed Monomi. He made an example out of Monomi by mercilessly executing her with his Monobeasts. Unlike Usami, Chiaki held no real authority, leaving her with no choice but to comply with Monokuma's violent demands. In turn, she was largely ignored, as her existence held no significance to him.

Throughout the killing game, Chiaki continued her morning routine: playing video games at the hotel lobby, briefly pausing before engaging in conversations, and then attending the group's morning gatherings. She also kept a diary that she secretly sent to Monomi regularly, with the contents detailing her observation of the supposedly Hope Restoration Program's participants.

She got along relatively well with her classmates, some even joined her daily gaming session from time to time. She admired the Ultimate Imposter's leadership, noticed small things such as being the first one who felt Fuyuhiko Kuzuryu's, the Ultimate Yakuza, absence from their meetings, and joined the girls-only gathering hosted by Mahiru Koizumi, the Ultimate Photographer. The bonus event could be triggered if Hajime gave Mahiru, a certain magazine. Chiaki invited Monomi, who returned to the group thanks to having spares, and shared some of the sweets with her.

Chiaki stayed composed when Monokuma tainted her original student Observer role as the traitor. She only showed signs of concern once he introduced the first motive, which potentially could have led to a murder from happening. She also feared that Monokuma would disrupt the all-night party hosted by the Imposter at the old building. She wanted someone to do something about it at first, but eventually volunteered herself to keep watch alongside Monomi outside the building.

Chiaki saw Fuyuhiko approaching the party venue as she just started guarding the entrance, leading her to wonder if he, contradicting his initial refusal to attend, had changed his mind. Fuyuhiko angrily denied her comment, claiming he was only going out for a walk, before aborting his plan and leaving the area.

The tone of her name, Nanami, meaning "seven seas" and Chiaki, meaning "one thousand autumns," was chosen to reflect her general image of possessing a kind of serenity that blends into the background so neatly, rendering her practically invisible. Her first name also incorporates the numeral kanji chi to mirror her creator's name, Chihiro. Both also share the same birthday, March 14th, which falls on White Day aswell as Pi Day.

The AI takes the appearance and age of the real-life Chiaki. She has blossom pink eyes with noticeable eyebags and short, messy dusty pink hair that almost seems spikey(though the locks on either side of her face curl inwards) which fades from a dark brownish pink to it's blossom color. She has spikey bangs that go to her nose and retains her real-life counterpart's signature white Gala Omega spaceship hairclip pinned to a lock of her hair as well as two other hairclips on the other side.

Chiaki normally has a quiet and sleepy disposition. She easily gets absorbed while playing video games to the point of ignoring her surroundings and forgetting her basic needs like sleeping and breathing. She stares off into space and pauses a lot amidst conversations so she has everything ready in her head before articulating it. She can instantly sleep anywhere and at any given time: while standing, swimming, reading, watching movies, riding a carousel, and the list goes on. She lucid dreams playing a different video game each time she sleeps and can perfectly retell them. Chiaki uses gaming terms and references often that sometimes she derails the conversation. At other times, she employs them to seriously convey her point.

Ignoring her unusual habits, Chiaki is usually stoic and unfazed about almost everything, even during her execution, which is very different from her real-life counterpart's much more human and painful reaction. She is also either too late to realize or does not mind when Hiyoko dubs her a "slowpoke gamer girl." She can stay calm and collected even under pressure, and only gets slightly livelier when discussing video games. However, she gets irked and pouty both when someone underestimates her intelligence and when someone fails to meet her expectations. In addition, she can be stern, bluntly honest, pushy, and ignore things she finds nonsensical, even at the expense of other people's feelings, mostly for good reasons. Chiaki's suspicion of Mikan during the third trial is a proof that she is willing to put aside their good relationships to reach out to the truth, reflecting her conviction that belief without doubt is nothing more than a lie. Even so, Chiaki second-guesses herself a lot, characterized by her adding "...I think" or "maybe..." at the end of her sentences. Hajime notices this frequently and prefers she can have a more confident-sounding catchphrase.

She is occasionally playful and tries to make jokes or play little pranks on Hajime to ease his tension, though sometimes they can be confusing due to her extremely flat deliveries. Her humor is mainly deadpan sarcasm, such as staring off into space before saying "too much info" after hearing Mikan's story about sleeping on top of Hajime. She also refers to Kazuichi and Akane's argument during the third trial as a "lover's quarrel," and even adds salt to Kazuichi's injury in the Funhouse by pointing out that not only was he rejected by Sonia, but he was also avoided. She also pretends to sound hopeless when facing a roadblock during class trials, before comically recovering one second later.

While Chiaki is extremely observant and analytical, she lacks knowledge of a variety of ordinary subjects due to the nature of her existence. For example, she doesn't know where milk comes from or what Girls' Day is. While she may understand the mechanics of how babies are made, dating etiquette and the concept of falling in love are foreign to her. Similarly, she lacks experience with living creatures, finding animals that don't mind being touched far easier to handle than people, who may anticipate or feel repulsion from contact. Due to this, she becomes very inquisitive and enjoys learning new things. She gets excited at the prospect of making sandcastles, taking part in suikawari, studying with someone, going on thrill rides so she can scream for the first time, and carrying out a real-life tactical espionage mission. She often equates unfamiliar activities to video games she knows, making the concepts feel more familiar to her. She loves to wander off inside her imagination, simulating video games scenarios.