Cepheus

Cepheus is king of the FM-ians. However, in his life, he had been extremely paranoid due to other FM-ians attempting to kill him in order to ascend to the throne. Gemini Spark then made use of the paranoia to get him to use Andromeda to destroy Planet AM.

Game History
When Planet FM received signals from Earth offering to form a BrotherBand, he interpreted it as an attempt to wage war on Planet FM, and so he started the assault on Earth which takes place over the course of the first MegaMan Star Force game.

At the end of the game, however, MegaMan defeated Andromeda, ending the conflict between Earth and the FM-ians. Cepheus realized the mistake he made, and decided to trust in Geo and Earth. Now Cepheus, with the 3 last AM-ians (Dragon Sky, Leo Kingdom, and Pegasus Magic) are trying to rebuild the AM planet.

In MegaMan Star Force 3, Taurus explains his presence on Earth is due to an order issued by Cepheus to hunt down two wanted criminals on Planet FM and AM - Corvus and Virgo. In the Extra Scenario, Cepheus appears to MegaMan and pleads him for help, as Sirius, who resides in a black hole, is threatening to destroy Planet FM. Apparently, the same black hole had already attacked Planet AM while he was rebuilding it with the three former Satellite Admins. After Sirius' defeat, Cepheus thanks MegaMan for saving Planet FM and leaves him with a final quest, telling him to "gather the 5 icons" (i.e. to unlock Crimson Dragon SP).

MegaMan Star Force (anime)
In the anime, Cepheus openly accepts the BrotherBand signals near the climax of the season, ordering the FM-ians to stop the assault on Earth and return home. He apologizes for the trouble caused. As Gemini Spark White attempts to use Andromeda to destroy the Earth in the last episode, Cepheus destroys Gemini, and wipes Patrick's memory of everything that happened while he was merged with Gemini. Cepheus also allows for Geo to speak with his father, who has been converted into EM waves by Omega-Xis. He physically appeared on-screen for only a short time, however.

Trivia

 * Cepheus, like many other FM-ians, is named after a modern constellation.
 * In Greek mythology, Cepheus was ruler of the Phoenician nation of Aethiopia.