Larcenous Lil first appears in The Lair of the Minotaur (1980) by Donald Brown. Lil, the adventurer's girlfriend, arrives in the town of Dunderhaven and attempts to burglarize the local castle, but is captured by some unsavory characters and held captive in the caverns underneath where the adventurer must find and rescue her. If the adventurer is female, Lil is replaced by a male character named Slippery Sven.
Lil reappears, again as the adventurer's significant other, in The Ice Cave (1987) by Jon Walker. Now described as "Lovely Lil", she's abducted by Broomhandle the Witch and must be freed. Lil's male equivalent in this adventure is Gorgeous George.
Lil's most recent appearance is in A Runcible Cargo (2012) by Thomas Ferguson where she can be found locked in a jail cell in the town of Dodge. If freed, she'll join the adventurer's party but driven by greed may attempt to steal the titular artifact. Ferguson notes, "the two of you have a long history, dominated by both Lil's backstabbing and the inevitable reconciliation." Lil is armed with a six-shooter pistol.