Zegapain

Kyo, an avid swimmer, is determined to keep his high school swimming club afloat despite being its sole member. When the student council threatens to shut it down, he enlists the help of beautiful Misaki, who agrees to participate in a promo video only if Kyo joins the Celebrum resistance organization and pilots a mecha to fight against the Deutera Areas, territories formed on Earth by the alien Gards-orm. As Kyo becomes embroiled in this new world, he begins to experience déjà vu, as if he's lived this life before.