L3-37 was an escaped synth assigned to X9-73 for recapture. L3-37 was mem-wiped, given the identity "Leo", and last seen at the Corvega assembly plant, where it has managed to evade capture twice using weapons skills learned from The Railroad.
L3-37 is mentioned only in Fallout 4.
Behind the scenes
L337 is one variation of the word leet; the method of forming words using numbers and other characters, also known as 1337 or leetspeak.