(covers information from several alternate timelines)
Manas, formerly Anderson Le, was a Human male who served as a Federation Starfleet officer until he was stranded on the planet Altamid. Le became the alien Manas after the use of energy transference technology severely mutated his body.
Le served as an operations division Starfleet officer during the 22nd century, serving on board the USS Franklin. In 2164, the Franklin was displaced by a wormhole while inside the Gagarin Radiation Belt. The ship crashed on the distant planet Altamid, with Le, Captain Balthazar Edison, and Jessica Wolff as the only survivors. (Star Trek Beyond)
On Altamid, Edison, Le, and Wolff found technology left behind by a previous civilization, including Swarm ships, a drone workforce, and an energy transference process that allowed them to "drain" other individuals to prolong their own lives. Using their newly acquired Swarm, Edison, Le, and Wolff captured scores of alien ships and drained their crews to survive, which also mutated their biology to resemble that of their victims. In time, Le became "Manas", suppressing much of his Human identity and even ceasing to speak in English.
During his time on Altamid, Manas aided Krall in corralling survivors of starships deliberately crashed on the planet to be used by Krall to extend his and his shipmates' lives through energy transference. At some point, Manas killed Jaylah's father as he helped his daughter to safety.
In the alternate reality, Manas joined Krall's forces in an attack on the USS Enterprise in 2263. Boarding the Enterprise, Manas was responsible for the deaths of numerous Enterprise crew members, at one point pursuing Lieutenant Commander Montgomery Scott through the remains of the engineering section and forcing the Starfleet officer to escape inside an advanced long-range torpedo.
Following the destruction of the Enterprise, Manas helped Krall round up the surviving Starfleet crew and hold them on the surface of Altamid. When Captain James T. Kirk and Jaylah stormed Krall's camp to free the Enterprise crew, Manas engaged Jaylah in hand-to-hand combat until Scott transported Jaylah back to the wreck of the Franklin along with Kirk and the rest of the Enterprise crew, leaving Manas to fall to his death from the top of a high structure. (Star Trek Beyond)
Manas was portrayed by Joe Taslim.
Make-up prosthetics for Manas were designed by Joel Harlow. "Manas clocked in at around nine pieces," Harlow remarked. "He was similar to Krall, because this entire crew has been feeding off the same alien DNA." (Cinefex, No. 148, p. 85)
A concept sketch of Manas can be found at TrekCore.