Kagari is a pale-skinned girl with long red hair, blue eyes and slender build. Although her similarities with Kurisu Makise are initially noted by Rintaro Okabe, there is no known connection between the two.
Due to the death of her parents during World War III in the Beta Attractor Field, Kagari became an orphan. Since the registers of her family had been lost during the war, her true name was lost. She first met Mayuri Shiina during the year 2028 as she was working at a children's orphanage, and was given custody of the girl.
As the connection between the two grew, Mayuri went to eventually adopt Kagari in the year 2034, becoming her new mother and giving her her new name.
However, during the time Kagari spent in the orphanage, she was a victim of numerous experiments lead by Alexis Leskinen, who was trying to test Amadeus' capabilities as military weaponry, under the pretence that she was being treated of PTSD.
In the year 2036, Kagari was forced to travel back to the past alongside Suzuha Amane to escape an upcoming attack. However, while they stopped in 1999 to stop the Y2K problem, the brain-washing she had been infected with resorted to taking advantage of Kagari's fear to lose her mother due to the worldline change to make her attack Suzuha and escape. After that, she went to reunite with the Alexis Leskinen of that time and was took by Stratfor to further experiment with her, who gave her the codename "K6205", a reference to Mozart's The Magic Flute.
- The name Kagari was given to her by Mayuri Shiina - a reference to "kagaribi（篝火?)", meaning bonfire -, with the wish that Kagari would be a shining light to everyone no matter how dark the worldline was.
- Kagari's surname Shiina means "oak, mallet" (椎) (shii) and "name, noted, distinguished, reputation" (名) (na).
- As with other elements of Steins;Gate 0, Kagari was first introduced in the Epigraph Trilogy.