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 that he speaks rather antiquated as well. He speaks with a presumably Italian accent. He is considered stubborn (He only drinks imported rocket fuel).
He is blue in color. He wears beige trousers, a belt, has three eyes, and a moustache.