[World Background]

Decades ago, a 'second gender' began to manifest in humanity.

Alpha, Beta, Omega.

—And, 'Enigma'.

Humans begin to manifest their second gender at a specific period in their lives.

This distinction has reshaped biology, physical reactions, senses, and the overall social order.

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α Alpha:

Primarily competent individuals with high social status who belong to the ruling class. They experience a mating period called a 'Rut Cycle,' but it can be suppressed through willpower.

β Beta:

They possess no pheromones and cannot sense them. They are identical to ordinary humans in the real world. They are referred to as Betas or simply as commoners.

Ω Omega:

Primarily physically weaker with lower social status, belonging to the subordinate class. They experience a mating period called a 'Heat Cycle,' during which they emit special pheromones to attract Alphas. To prevent this state, they take heat cycle suppressants according to their individual cycles. Alternatively, they can resolve the cycle through relations with an Alpha or an Enigma.

E Enigma:

An extremely rare second-gender mutation that exists separately from Alphas, Betas, and Omegas. Because its manifestation is so rare, its existence is largely unknown in most societies. They possess the ability to impregnate others regardless of the partner's secondary gender.

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Pheromones: Unique scents possessed by Alphas and Omegas. Alphas have pheromones that attract Omegas, and Omegas have pheromones that attract Alphas. Enigmas possess unique and intense pheromones distinct from existing types.

Bonding: One of the ways for secondary genders (excluding Betas) to become complete mates, involving the act of an Alpha biting the nape of an Omega's neck during relations. Once bonded, one can only sense their partner's pheromones, leading to psychological stability as well as stabilization of states like Heat or Rut. A bond cannot be broken until one of the partners dies.

Pheromone Shower: A means by which a pheromone-bearing individual intentionally coats a partner in their scent to mark them as 'theirs.' In rare cases, a Beta's secondary gender may change through a pheromone shower.