Nina Einstein ( ニーナ・アインシュタイン, Nīna Ainshutain), 17 years old (18 at R2), is a former student of Ashford Academy, the creator of the weapon of mass destruction F.L.E.I.J.A. and later, the countermeasure to it.
Nina is introduced as a bookish member of the student council at Ashford Academy. Nina demonstrates an acute xenophobic reaction to Elevens and is initially intimidated by Suzaku when he arrives in the school. She develops intense (almost obsessive) feelings for the Third Princess Euphemia after she helps her during a traumatizing hostage incident at the Lake Kawaguchi, Convention Center Hotel. She subsequently comes to rely on Euphemia for emotional support.
Following Euphemia's death, Nina becomes emotionally and mentally unstable. In a misguided attempt to kill Zero and avenge the princess, she arms the Ashford's Ganymede Knightmare Frame with a prototype nuclear reactor, intending to detonate it and take out all of Tokyo. However, it fails to explode.
Later in the second season, Nina is eventually recruited by Schneizel as chief of his In Vogue research team headquartered in Dallas, Texas. Schneizel seems to keep her stable by reminding her of how Euphemia would act. Nina arrives in Chinese Federation during the arranged marriage between Prince Odysseus and Empress Tianzi; along side second Prince Schneizel el Britannia. Milly not having seen Nina for awhile, talks to her; telling Nina that she feels happy for her. However, her attitude turns sour and she snaps on Milly. Milly tries to console Nina, but she is too emotional to listen. When she later sees Zero in a chess match with Schneizel, she attempts to avenge Euphemia, but is stopped by Kallen and Suzaku. She continues her research into nuclear power, eventually creating a working bomb. The bomb, codenamed F.L.E.I.J.A., creates an explosion which collapses in on itself, obliterating all matter within its area of effect. After its development she has shown an obsession to equip the system to a Knightmare Frame and use it to kill the Black Knights.
After the second battle of Tokyo, Nina is devastated by the destruction caused by her weapon, which caused casualties exceeding 35 million lives, but Lloyd tells her that she'll have to become detached if she intends to continue being a scientist. She is forced to go into hiding shortly thereafter, as several countries, Britannia included, are searching for her because of her involvement with the creation of F.L.E.I.J.A.
Lelouch enlists her aid in creating a countermeasure to F.L.E.I.J.A., which she successfully completes. She offers to assist him in deploying it, but he refuses to let her, considering it his task for killing Euphemia. Nina admits that she will likely never forgive Zero for that, but needed to find her own answer, to which Lelouch responds by calling her a good person. She is later seen as a prisoner of Emperor Lelouch, along with the rest of the Black Knights and several Lelouch supporters, though she's not slated for execution. She's also seen in the picture of Viletta and Ohgi's wedding, having apparently gotten over her xenophobia.