Zegapain is a 2006 post-apocalyptic sci-fi mecha anime series by Sunrise with manga and video game adaptations. A sequel for the series was announced August of 2023.
The anime series follows Sogoru Kyou, a high school boy living a normal day to day life in the city Maihama. A member of the school's swim club, he one day meets the girl Misaki Shizuno at the school's pool and is startled to find initially he's the only person who can see her. As his clubs is at the risk of being closed, Kyou decides to recruit Shizuno to make a promo video for the club to draw in new members with the help of his childhood friend, Kaminagi Ryouko. Shizuno agrees, but on the condition that Kyou pilots a mecha from what appears to be a video game. Kyou agrees, but finds himself seemingly pulled into the video-game-like world where the the organization Celebrum is fighting a war against enemies called Gards-orm.
Franchise premise: The franchise as a whole revolves around the ongoing war between the organization Celebrum and the invaders called the Gards-orm. The Gards-orm are part of a plan developed by the iAL Company, who developed a means of digitizing the human mind and uploading them to exist within a quantum server. The CEO of iAL Company envisioned this as a means of advancing humanity to the next stage of evolution and to force his vision developed a virus that lead to the extinction of organic humans. While most of humanity was wiped out, many still survived by being digitized onto quantum servers independent of iAL Company's control. Some of the survivors from these servers became able to leave upon realizing the world they lived in on the servers were simulations, these "awakened" survivors created the organization of Celebrum. Using stolen technology they were able to produce weapons to both defend their servers and to stop the Gards-orm from re-terraforming the Earth's surface.