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Baby Luigi (also called Billy in R64: Mario Goes Shopping) is a minor character in the SMG4 series.
He is the baby form of Luigi.
He later appeared in R64: Mario Goes Shopping, where X calls him Billy, saying to him that he can't play in the street. Mario then runs over X, prompting Baby Luigi to say "Dafuq?". Later, X crawls back to warn him not to play in the street, only to be hit by Mario's car again. Baby Luigi responds "Surprise, motherfucker."
The next time that he appeared was in The (TOTALLY ACCURATE) Documentary of Mario where he meets his brother, Baby Mario in an orphan.
In SM64: Meet the Luigi, he was taken by a vulture and got scared.