R4-D12 was an R4 astromech droid owned by the professional racer Marcus Speedstar. The droid assisted Speedstar in navigating his ship during races. In 34 ABY, the astromech took part in the Platform Classic, a major race held on the Colossus refueling station on Castilon that was ultimately won by Speedstar.
During the New Republic era, R4-D12 and the Esral'sa'Nikto Oplock were members of the racing champion Marcus Speedstar's team. The droid and Speedstar accompanied Speedstar when they visited his estranged older brother Jarek Yeager's repair station aboard the Colossus. While Yeager's Team Fireball were friendly to the visitors, Yeager refused to speak with Speedstar, whom he blamed for the deaths of his wife and daughter.
During the Platform Classic, R4-D12 sat in the astromech socket of Speedstar's racer Galaxy's Glory. Yeager taunted Speedstar by claiming that D12 looked worried. Speedstar responded by making fun of Yeager's astromech droid Bucket. During the race, Yeager complimented D12's loyalty to Speedstar. During the race, Yeager reconciled with his brother after realizing that Speedstar wanted to save Oplock, who had been taken hostage by the Guavian Death Gang for unpaid debts. Yeager allowed Speedstar to win.