The FM-ians are the main antagonists of Mega Man Star Force. They are a race of EM Beings from the Planet FM who come to Earth with the intention of wiping out humanity after interpreting BrotherBand transmissions as a sign of hostility.
In the past, the High King Cepheus became paranoid of the neighboring Planet AM and had it destroyed by the FM-ian war machine Andromeda due to seeing it as a threat.
The FM-ians first came into contact with humanity after receiving a BrotherBand transmission from them. However, Cepheus saw the transmission as a declaration of hostility and was persuaded by his adviser Gemini to go to war with humanity.
However, after the destruction of Andromeda, Cepheus learned what a BrotherBand really was and ceased hostilities with humanity. Various FM-ians would go on to appear in later games in the series as both enemies and allies of the heroes.