Lily was born at some point after her father's future self gave a lecture to his younger self after noticing how he was ignoring Clarissa on her birthday while they were in Washington DC. Younger Martin took his older self advice to heart and conceived Lily during that window of opportunity.
In Kindergarden, she ended up getting in a fight with her teacher for wanting to sing the periodic table instead of the alphabet. Her father was called in, but was amused by this.
When Lily was in seventh grade, she and her father stayed up all night building her science project, with Martin giving her coffee to stay awake, much to the annoyance of her mother.
Helping her father with the Dominators
After seeing visions of her, Martin Stein and Caitlin Snow visited Lily at their home. She expressed her love for him, before the two rushed away at the behest of Martin. Not long after, Caitlin invited Lily to S.T.A.R. Labs to assist with her knowledge in nanotechnology. Despite attempting to work on a solution to the Dominators, Lily went to leave, to being unnerved by Stein's apparent neglect. Catching her on the way out, he apologized, admitting that his life had become complicated. Sparked by this, they realized that the reaction of their technology required greater complexity to work. Fixing this issue, they used the devices to cripple the Dominators.
Lily has been shown to be a very cheerful, warm, and friendly young woman with a rabid interest in science, a trait shared with her father. Lily appears to use science not only as her passion in life, but also serves as the catalyst for her relationships with her father, both professional and familial.