Cepheus

Cepheus is an EM Being and the king of the FM-ians. Throughout 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 has made use of this paranoia before, successfully urging Cepheus to use the Andromeda to destroy Planet AM.

MegaMan Star Force
When Planet FM receives signals from Earth offering to form a BrotherBand, Cepheus interprets it as an attempt to wage war on Planet FM, and begins an assault on Earth which takes place over the course of MegaMan Star Force.

However, when MegaMan defeats Andromeda, ending the conflict between Earth and the FM-ians, Cepheus realizes the mistake he has made, and decides to trust in Geo and Earth. Cepheus then begins to rebuild Planet AM along with the three Satellite Admins.

MegaMan Star Force 3
In the extra scenario of MegaMan Star Force 3, it is revealed by the Satellite Admins that an EM Being named Sirius is guiding a black hole towards Planet FM in the hopes of consuming it for his "collection."

Cepheus appears before MegaMan on the Astro Waves of Planet FM, pleading him for help. He informs MegaMan that the same black hole has already swallowed the Planet AM that he and the Server Admins have tried to rebuild, and states that it will consume the FM-ians' planet in less than one day.

MegaMan goes on to defeat Sirius, after which Cepheus thanks MegaMan for saving Planet FM, reminding him that they will always be Brothers.

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.

Name
Cepheus is the name of a modern constellation, one of the 48 constellations listed by the 1st century astronomer Ptolomey, and remains one of the 88 modern constellations.

In Greek mythology, the second Cepheus, King of Aethiopia is the father of Andromeda.

Trivia

 * Cepheus is one of the only bipedal EM Beings that is not an EM Wave Change who has clearly-defined legs.
 * Cepheus' "crown" resembles Geo's collar.