Sekirei are superpowered beings with a different code similar to a human's. The Sekirei spaceship crashed on Earth in 1999 on Kamikura Island , and was found by two students, Hiroto Minaka (future MBI chairman) and Takami Sahashi (future MBI head researcher and Minato's mother). On board, the pair had found 108 life-forms, described as "One pillar and 107 baby birds".
The physiology of a Sekirei is similar to humans. They have similar requirements such as the need to eat food, drink water, and breathe oxygen. They are also also capable of reproducing with the human species; thousands of years in the past, some Sekirei were able to conceive offspring with their Ashikabi. Due to this, the genes of Sekirei, while recessive, are still carried by nearly half of the world's population. However, they also have significant differences that set them apart: Sekirei have greater strength, speed, and stamina than is possible for humans. In addition, each Sekirei has a that is the source of their power and very existence; Sekirei who lose their core cease to function.
An important rule for every Sekirei is that the winner has to guard the corpse of a Sekirei that is terminated until arrives and recovers the body. In the manga, if a Sekirei does not follow this rule and refuses to show their respect to the loser, other Sekirei might try to punish them. Musubi stated that the reward for the last remaining Sekirei of the game (the ) is for them to be with their favorite person - their Ashikabi - forever.
Minato soon learns that being the partner of a girl is not all fun and games, especially when several other Sekirei choose him as their Ashikabi, each also forming a contract with him. Now, Minato must find a way to survive both the life-threatening battles of the Sekirei Plan and his partners' fierce competition for him.