Galen Walton Erso is a supporting character in Rogue One: A Star Wars Story. He is the father of protagonist Jyn Erso and a brilliant scientist who made his name among the Galactic Republic's scientific community with his research into kyber crystals. With the rise of the Empire, however, Galen found himself reluctantly working on weapons applications for kyber crystals and played a pivotal role in designing the superlaser for the Death Star. He was portrayed by Mads Mikkelsen.
Following the Clone Wars, Galen had taken up a lucrative research position with Project Celestial Power, an Imperial program that was supposedly meant to provide sustainable energy for planets that had been left devastated in the aftermath of the war. Working under his wartime friend Orson Krennic, Galen was led to believe that he was working to build a better and brighter future for all peoples of the galaxy. As the years went on, however, he discovered that the Emperor was planning to use his work for far less benevolent purposes. With the help of known rebel Saw Gerrera, Galen, along with his wife and daughter, escaped from Coruscant and went into hiding on the distant world of Lah'mu.
The Ersos remained hidden for a few short years on a private homestead, eking out a peaceful life as moisture farmers. However, in 13 BBY, Krennic found them and encouraged Galen to return to the Empire's service. Galen attempted to lie his way out of the situation, but Krennic saw through the deception. Galen's wife Lyra threatened Krennic with a blaster, but failed to kill him and was in turn killed herself by Krennic's Death Troopers. Galen then submitted and returned with Krennic to continue his work on the Death Star project. As for his daughter Jyn, her escape and rescue had been arranged beforehand and she was taken to safety by Saw Gerrera.
Over the next thirteen years, Galen reluctantly continued his research into kyber crystals. By this point, the Death Star project had come along so far that it could be completed with or without him, so Galen's refusal to work would have no meaning. Instead, Galen made himself indispensable and focused his efforts on development of the Death Star's power regulation and distribution systems. While Krennic and the other Imperial scientists believed that Galen had resigned himself to the role of a defeated, obedient servant, Galen was never truly loyal. He was intentionally designing a fatal flaw into the battle station's reactor system, minute so that it would go unnoticed unless one knew where to look. Any pressurized explosion to the component would set off a chain reaction that could completely destroy the station. Hoping that his daughter was still alive and safe from Krennic, Galen codenamed the Death Star's structural readouts "Stardust," the name he called Jyn as a child.
Stationed on Eadu, Galen eventually convinced Imperial pilot Bodhi Rook of the true horrors of the Death Star. Overcome with a crisis of conscience, Galen entrusted Bodhi with a message to be delivered to the Rebellion, and hopefully his daughter, informing them of the station's vulnerability.
By 0 BBY, rumours soon began to spread throughout the galaxy of a new Imperial superweapon. Shortly before the Battle of Yavin, the Rebel Alliance received word that Saw Gerrera had a message from Galen Erso pertaining to the superweapon. Recruiting Galen's estranged daughter, the Rebellion tasked Jyn with traveling to the Force-sacred world of Jedha to secure a meeting with Gerrera, a former friend of hers, and retrieve the message.
After traveling to Imperial-occupied Jedha and watching her father's message, Jyn was led to believe that the Rebellion sought to extract her father from Imperial custody, while in reality, Intelligence officer Cassian Andor was under direct orders to assassinate Galen at his first opportunity by General Davits Draven, who was fearful of what other weapons Galen could create for the Empire. While on a covert mission to Eadu to assassinate Galen, Cassian lost the will to pull the trigger after seeing Galen defend his fellow researchers from execution after Krennic realized a transmission had been sent from the facility. Threatening to shoot all researchers if the traitor did not reveal themselves, Galen announced that it was he who had sent the coded transmission, leaving his place by Krennic's side to plead for his colleagues' lives. Galen's words were met with deaf ears, as Krennic ordered the researchers executed. Angered by Galen's betrayal, Krennic hit Galen across the face. Before any further action could be taken, a Rebel Alliance strike force assaulted Eadu owing to a communications blackout from Cassian and his forces on the planet, with Draven fearing the worst had occurred. A proton bomb from a passing Y-wing assault bomber struck the platform where both Galen and Krennic were standing, sending them flying to the ground. Jyn Erso, who had watched the exchange from a distance, ran to the platform where her father lay dying, revealing herself to him in his final moments. After a few brief words with his daughter, Galen died from his wounds in Jyn's arms.