R.O.B. was the new robot character added as a playable racer in Mario Kart DS.
R.O.B. stands for Robotic Operating Buddy, a Nintendo accessory originally released for the Nintendo Entertainment System in the 1980s. His appearance in Mario Kart DS was a notable deep cut: unlike most racers, he came from Nintendo hardware history rather than a conventional Mario character role.
In Mario Kart DS, R.O.B. is one of four unlockable characters, alongside Daisy, Dry Bones, and Waluigi. The game’s initial roster draws heavily from earlier Mario Kart entries, so R.O.B. stood out as a completely new addition to the playable lineup.
The character is also associated with a special vehicle, the ROB-BLS, and belongs to the heavy character class. Shy Guy can appear in multiplayer Download Play, but that does not make him the robot racer introduced to the main playable roster. King Boo and Dry Bones likewise belong to later or returning character lineups rather than being the answer here.