Galen Walton Erso is a human crystallographer whose work on kyber energy amplification makes the Death Star's superlaser possible, and whose deliberate sabotage of the reactor module makes its destruction possible. He first appears in James Luceno's novel Catalyst: A Rogue One Novel (Del Rey, 2016), published ahead of the film, and is played by Mads Mikkelsen in Rogue One (2016).
The long con
The character's defining act takes fifteen years and consists almost entirely of appearing to cooperate. Taken by Orson Krennic after his wife Lyra is killed, Galen concludes that suicide or refusal would simply get him replaced by someone who would build the weapon correctly. So he stays, works, becomes indispensable, and hides a single structural weakness in the reactor — a flaw small enough that no one would find it and total enough that one shot would end the station.
The message
He gets it out through the defecting pilot Bodhi Rook, addressed to Saw Gerrera and, through him, to his daughter Jyn. The holomessage is the hinge of the film and it is mostly an apology. He calls her Stardust, which is also the project's Imperial codename — the one detail that lets the Rebellion find the plans on Scarif.
Retroactive engineering
It is worth being clear that this character exists to solve a forty-year-old plot complaint: that the Death Star's exhaust port is absurd. Rogue One's answer is that it is absurd on purpose, because a coerced scientist put it there. That is a rare case of a retcon improving the original film rather than merely patching it.