Ronaldo is a blue alien who frequents the Shooting Star Milkshake Bar to drink rocket fuel. He is seen with Ferb collecting some rocket fuel in his tea cup from a petrol pump.
He is rather posh as he holds his tea cup with his little finger out and speaks rather antiquated with a (presumably) Italian accent. He is also considered stubborn, as he only drinks imported rocket fuel.
He is blue in color. He wears beige trousers, a belt, has three eyes, and a moustache.