A Unknown is someone with the ability to use power. Its unclear why theres a fight but most fights starts with Dark unknown. There appears to be no rules at the current moment and the only one who knows anything is Noui. The story of unknown first started at blogger then moved into Visual Novel Game since then the whole story is now a Visual Novel with many different events. Its unclear how these powers are activated but it has lean to the emotion and thought. Anyone can be a Unknown as told in 'The Unknowns saga', a ghost, a wild person, normal person, elf, etc.
In a cutscene, Noui told that the Unknowns were fighting since they were born. There is a possible link that many of the Unknowns have been reborn. Many believe that Unknown means a pokemon named "Unknown" which is not true. Most Unknowns has a backstory before entering the 'Unknowns Saga series', a demo game called 'The Unknowns chronicles' shows current Unknowns back stories but not releasing new information on a Unknown. Noui believes that Angels power is too great and never tries to give him a lot of info, as in Dark unknown who always wants to kill Unknown #1 so he can rule the world.