context: Teenager Shinji Ikari is the pilot of Evangelion Unit 01, one of several giant cyborgs designed to fight hostile supernatural entities called angels. Distraught over the death of his friend Kaworu Nagisa, Shinji visits fellow pilot Asuka Langley Soryu in a hospital and masturbates to her comatose body.
  The shadowy committee SEELE has discovered that commander Gendo Ikari intends to use NERV, the paramilitary organization that deploys the Evangelion units, for his own plans. SEELE dispatches the Japanese Strategic Self-Defense Force (JSSDF) to seize control of NERV, killing most of the staff. NERV major Misato Katsuragi orders Asuka moved to the cockpit of Evangelion Unit 02 and placed at the bottom of a lake, then rescues Shinji from JSSDF troops. Determined to have Shinji defend NERV, Misato brings Shinji to Unit 01's bay doors, but is mortally wounded in the process. Misato implores Shinji to pilot Unit 01, kisses him, and forces him into the elevator. Shinji discovers that Unit 01 is immobilized in JSSDF bakelite.
  After concluding that NERV's defeat is inevitable, Gendo retrieves the Evangelion pilot Rei Ayanami. He plans to use her to initiate Third Impact, a cataclysm which will kill everyone on Earth, and reunite with his deceased wife Yui. Attempting to stop him, NERV scientist Ritsuko Akagi sends a computer command to destroy NERV. However, Casper, a computer core modeled on Ritsuko's mother, overrides her command and Gendo kills her.
  Inside Evangelion Unit 02, Asuka overcomes her trauma and re-activates the unit. She destroys the JSSDF forces, but SEELE's new mass-produced Evangelion units dismember her and Unit 02, leaving her for dead. Breaking free of the bakelite, Unit 01 ascends above NERV headquarters. From the cockpit, Shinji witnesses the mass-produced units carrying the mutilated remains of Unit 02 and screams.
  Gendo attempts to merge with Rei, who has the soul of Lilith, an angel hidden beneath NERV headquarters, to begin Third Impact. Having merged with another angel, Adam, Gendo will become a god if he merges with Lilith; however, Rei rejects Gendo, absorbs Adam and reunites with Lilith, and her body grows to gargantuan size. The mass-produced Evangelion units pull Unit 01 into the sky and crucify it, beginning the ritual to initiate Third Impact.
  After several dreamlike contemplations, including a fight with Asuka, Shinji decides that he is alone and everyone in the world should die. In response, Rei/Lilith dissolves humanity back into primordial soup, reforming the souls of humanity into a single consciousness. Initially accepting this new state, Shinji feels that there is no happiness in the real world. However, he rejects it when he realizes that life is about experiencing joy, as well as pain. Rei/Lilith dies and Asuka and Shinji rematerialize in an apocalyptic landscape. Shinji begins to strangle Asuka, but when she caresses his face, he stops and breaks down in tears.
question: Who are trying to prevent the occurrence of Third Impact?
answer: Ritsuko Akagi