1. **Key Awakening:** At the age of eight, a person’s soul resonates and awakens a unique Key. Keys take many forms, such as crystals, runes, objects, or living creatures, and determine which otherworld the bearer can travel to.
2. **World Ranks:** Otherworlds are ranked from E to S according to the level of power they contain, the strength of their rules, and the advancement of their civilizations. Examples include Norse mythology (A-rank), the Marvel Universe (S-rank), the DC Universe (S-rank), the Kamen Rider Universe (B-rank, with enormous potential), Saint Seiya (A-rank), the Cthulhu Universe (S-rank, extremely dangerous), martial arts worlds (C-rank), and wasteland worlds (D-rank). A world’s rank indicates the “limit” of the power that can be gained there and its “danger level,” but it is not absolute.
3. **Gaining Abilities:** A Key opens a one-time, brief journey to another world, where the flow of time may vary. Through experience, contact, study, or fusion in that world, a person gains a unique ability, item, or body of knowledge. Its strength, form, and potential are closely tied to the individual’s aptitude and experiences, the Key’s characteristics, and the world’s rules. Afterward, the Key “cools down,” becoming a symbol of identity and a source of power.
4. **Uniqueness:** Even when several people travel to the same world, such as different versions of Norse mythology, they will gain entirely different things because they arrive at different times and places and have different encounters.
5. **Different Power Systems:** The rules governing power differ between worlds and may clash or complement one another, such as divine power, technology, biological energy, and Cosmo. Combining them is both a challenge and an opportunity.
