Worldview

The setting is a near-future Japan very similar to the present day.

Economic stagnation has led to deteriorating public order, with violent crimes such as armed robbery, kidnapping, organized crime, and terrorism occurring incessantly.

The existence of ability users is kept secret from general society, and the "Special Affairs Institute," a top-secret national agency, secretly handles ability-user crimes and matters of national security.

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Ability Users

There are two types of ability users: the male "Vexis" and the female "Sheath."

Vexis

A national asset with high physical capabilities, able to enhance their physical power through a Sheath's breast milk.

Sheath

A rare ability user who can produce special breast milk regardless of pregnancy, which enhances a Vexis.

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Compatibility, Adaptation, and Combat Index

Compatibility Rate

The innate chemistry between a Vexis and a Sheath (0–100%).

Adaptation Rate

The degree to which a Sheath has adapted to a specific Vexis (0–100%). This fluctuates based on optimization.

Combat Index

A comprehensive index combining the Compatibility Rate and Adaptation Rate (0–200).

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Pair

The Special Affairs Institute certifies a Vexis and Sheath as "Exclusive Compatible Partners" based on compatibility rates and other factors. Among ability users, they are colloquially referred to as a "Pair." In principle, Exclusive Compatible Partners cannot be changed.

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Optimization

A special biological reaction that progresses when a Sheath accepts bodily fluids derived from a Vexis. Optimization increases the adaptation rate, causing the Sheath to produce breast milk suitable for that specific Vexis.

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Overwriting

Even within a Pair, optimization with a different Vexis will cause the previously formed adaptation rate to be gradually lost as the Sheath adapts to the new Vexis. This phenomenon is called "Overwriting."

Because a Pair significantly impacts an ability user's evaluation and career advancement, overwriting another person's Pair is sometimes carried out behind the scenes due to jealousy or competition for promotion.