The universe of Rebus is primarily designed for D&D 5e. Any of the standard player race options in officially published books are allowed, as well as any homebrew statblocks within the verse. Any homebrew races, items, features, spells, or abilities are allowed at the discretion of the dungeon master.
Typically, PCs will be mortals hailing from either the realm Slavalon or Shometria. If the DM is running a godly or higher tiered campaign, the player characters will hail from Aephotora or may potentially have already fallen and sprouted in Aebbiran.
There are many secrets in the verse of Rebus. Either intentionally hidden or yet to be discovered.