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Billy's tattoo is one of three references to the band Queen: "Mother Love" is the name of a Queen song, the same as "Made in Heaven" (from Claire's jacket in Resident Evil 2, as well as one of Chris's alternate costumes) and "Let Me Live" (from Claire's Jacket in Resident Evil CODE:Veronica)
Billy has two different official heights. He is shown in-game to be 5'9" as seen here, whereas the Japanese website for Resident Evil Zero lists him as being 5'11".
The Nintendo 64 version of Resident Evil 2 features a file named Rebecca's Report. In it, Rebecca details finding Billy's body (though his last name is as "Koen") in the woods with the MP vehicle (seen at the beginning of Resident Evil Zero). She also states that Bravo team was forced to abandon his body once they were attacked, and it was presumed lost. This file was added in to the N64 version of Resident Evil 2 to foreshadow the then upcoming Resident Evil Zero which was also being developed for the N64. However, it was later cancelled and development shifted to the GameCube.
In the original build for Resident Evil Zero for the Nintendo 64, S.T.A.R.S. was unaware of Billy Coen's status as an escaped criminal, and only wanted to track him down to question him about his presence near an overturned vehicle. It was only after Rebecca Chambers found handcuffs on him that this would have been revealed.