Being a Bleach-based RPG, Halcyon Days has several factions that match those in the original series.
Spiritual dwellers of Soul Society in a never-ending war agains the Hollows. Based around the Gotei 13, a force of Thirteen Divisions, each with a set role to perform.
Former Shinigami who've obtained Hollow powers and gone rogue. They are split into four echelons, each with a set purpose and named after figures from Greek mythology.
Humans who use spiritual bows to eradicate Hollows. They have no set groupings, instead wandering free and fighting Hollows wherever they may be.
Humans who have gained spiritual abilities from encounters with Shinigami or Hollows. Split into different groups be it the students of Tokyo Nishi Koutougakkou or the scientists of the Cereberus Corporation.