She was the mate of Terry before his death.
Over 10 years ago, Lucy lived happily with her mate. She was a good and loyal wife. However, then Terry joined the Ōu Army which mission was to defeat monstrous bear Akakabuto. Terry died during his mission and thus never returned. Lucy heard the sad news and grieved for long, falling into depression and shutting herself from others. She lost interest in sociality and talked only rarely, feeling envious of the happy dogs around her and turning more and more bitter. She became mentally ill though the other dogs didn't seem to notice this.
Lucy was allowed to live close to Gajō. One day, Gin's pregnant mate Sakura came to Futago Pass. Lucy hated how happy she was and aimed all her bitterness at her. In a month, Lucy turned more social to become "friends" with Sakura and gain her trust. Tony was suspicious of her change of behavior but Smith assures it's nothing. While the others are on a patrol, Lucy lies to Sakura by telling her that her mate Gin has been fatally wounded in a landslide in Northern Alps and might be dying soon. Shocked, Sakura wants to go help her mate and Lucy guides her inside a truck. Lucy breaks into tears as the truck drives away, Sakura inside.
Smith returns to find Sakura gone and Lucy crying and asks for an explanation. Lucy breaks down, admitting her jealousy of Sakura and wishing they all would choke into their happiness. She promises to tell Smith what she had done to Sakura if he would kill her afterwards. It is unknown if Smith did kill her or not. If Smith refused to kill her, it's likely she committed suicide.