Final Fantasy: Unlimited S01

Years ago, the opening of an interdimensional portal brought two monsters into the sky above the Sea of Japan, visible to all the surrounding countries. After destroying the naval fleet sent to deal with the issue, the two monsters turned against each other. Scientists Joe and Marie, who were watching the battle from Japan, became interested in the subject. After returning to Earth, they compiled their information and published their academic findings in a book called "The Day of Succession". Ai and Yu Hayakawa, who are twins, decide to leave their family and research the subject.