The Wash bot was a robot replica of Hoban Washburne introduced in the Return to Earth-That-Was arc.
The robot was created by the Alliance and Blue Sun Corporation that previously created robots based on Malcolm Reynolds. It was made as bait for a trap meant to capture Zoë Alleyne Washburne, but the robot possessed Wash’s personality and some of his memories, so it behaved as Wash would, protecting Zoe and getting her to safety.
The Wash bot was with the crew when they went through a portal and landed on Earth-That-Was. It led the crew to the regional headquarters of Washburne Industries, an appliance manufacturer run by Wash’s ancestors. The robot helped the crew as they battled scavengers that had also reached Earth through the portal and hostile Earthers,
Zoe knew the robot wasn’t really Wash, but the shared memories and personality and likeness led her to tell the robot that she didn’t want to see it die. Emma also liked it due to the similarities to her father. Wash bot said it was just trying to find a place of its own in the ‘verse. In the end, it decided to stay behind on Earth while the Serenity crew returned home. Someone was needed to stay behind to ensure the portal was closed from both sides. Zoe originally planned to stay behind with Emma, but changed her mind. The Washbot decided he would be the one, and left in a shuttle with Jefa after Serenity lifted off.