Sehto's culture and mythology is a bit of a patchwork.
The island has seen settlers from three separate cultures over the course of its existence:
- the Kyza, from 300-800
- the Xhosa, successors to the Kyza and sole occupants of the island from 800-1400
- the Sehtoan, chiefly settlers from Johto arriving 1400+
The Sehtoan culture is mainstream since about 1400, but has experienced a large cultural shift away from its Johto roots. The Xhosa are an integrated minority on the island at this time.
Mythologically, Sehto is almost entirely atheistic. Due to the existence of Ghost and Psychic pokémon, there is some belief in the metaphysical, but not as a coherent quasi-religious framework. Exceptions are some groups that follow Xhosa mythology (often Kyza-inspired, though the Kyza themselves are part of the mythology, making the line between adopted and invented mythology very blurry indeed), and Pyu Ivvi's superstitions that rank around tales of trolls, near-elemental creatures of dubious and capricious personality responsible for weather as well as good or ill fortune. Even these two most visible mythological frameworks have little impact on social interactions, however - and perhaps more interestingly, at the end of the day, you can find interesting stories in just about anyone on Sehto, ranging from the plausible to the far-fetched, painting a lush picture of wildly differing spiritual or quasi-spiritual beliefs.