Epsilon, known as Ipsilon in Japan, is the "Unknown Energy Manipulator: Test Model" from Mega Man X: Command Mission. He serves as the leader of the Rebellion Army and acts as the game's antagonist up until the final chapter.
While exploring the Lagrano Ruins, X can find a pod used during Epsilon's development. His pod details that he was the only Reploid to survive the Supra-Force Metal infusion experiments, giving him heightened intelligence and strength capabilities. All other volunteers in the experiment either went mad or perished outright.
Before the story, Epsilon masterminded the revolt in Giga City and became the leader of the Rebellion Army's plans. This disturbance causes the Maverick Hunters to deploy X, Zero, and Shadow to the island for an investigation. However, Epsilon manages to turn Shadow to his side before the mission and gets him to lead X and Zero into an ambush with the rest of the cadres. Epsilon tries to convince the two Hunters to join his cause, but they both refuse and escape, albeit separated. Although Ferham wants to give pursuit, Epsilon orders her to stand down.
Throughout the rest of the game, Epsilon makes attempts to control Giga City's resources while crushing the Resistance that opposes him. Realizing he needs force to make others listen to his ideals, he resorts to creating a missile armed with Supra-Force Metal to keep his foes at bay. He directly orders his cadres to carry out these missions on multiple occasions.
Eventually, X and his team trace Scarface's location, leading them to Epsilon's hideout in the Grave Ruins Base. Epsilon is prepared to fight them, but Scarface takes his place, urging him to flee. After Scarface falls in his name, Epsilon refuses to run away and takes them on himself, only to lose. He then states that his options for achieving his goal were very limited, due to his group being labeled Mavericks so quickly. As he tries to continue fighting, X brings him down with his buster.
After the end of the game, Epsilon was deferred of all Maverick charges posthumously, most likely due to the restraint he showed in not killing innocents, and the fact that his whole Maverick status came about as a result of the traitorous Redips' command.
And he will be voiced by Frank Welker in the English dub and he is reprised by Yukitoshi Hori in the Japanese dub.