- "It's not your fault I'm a mistake."
"I have made many mistakes in my life, Lily. But you are most certainly not one of them."
- —Lily Stein and her father, Martin Stein[src]
Dr. Lily Stein is a scientist, and the daughter of the late Martin Stein and Clarissa Stein, the girlfriend of an unnamed man, and the mother of Ronnie. After a meeting between Martin and his younger self, the timeline was altered, resulting in Martin and Clarissa choosing to have a daughter. Lily, not previously existing in the other timeline, was a time aberration, but her father and the Legends have allowed her to stay in existence.
Lily was born to Martin and Clarissa Stein sometime after 1987. Lily's existence is the result of Martin's older self from 2016, who lived a life without having children, lecturing his younger self from 1987 on his neglect of Clarissa in pursuit of his scientific achievements and urged him to be more attentive to his wife. Martin's past self took this advice to heart more than his older self expected, which resulted in Lily's birth.
In Kindergarten, Lily 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 the Stein family home. Lily expressed her love for her father 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. Frustrated by her struggles to find a solution to the Dominators, Lily decided to take a break, unnerved by Stein's apparent neglect. Catching her on the way out, Martin 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.
After they parted, more events from Lily's childhood began to come to her father in the form of memory flashes and Stein considered each one of them a gift.
Learning the truth
When the Legends couldn't decipher the mystery of the medallion, Stein went to Lily thinking she might be able to find out more. She met her father and Jax at Jitters where he knew he would find her. Jax was surprised to actually meet Lily and even more surprised that she was who Stein had in mind to help them. Lily brought out a portable ionic scanner which Stein was astonished to see in a portable version and Lily told him that she designed it herself. She was able to activate the medallion and Stein quickly put it away and decided to bring her back to the Waverider. She was astonished to see the ship while the Legends were surprised to meet her. She and Ray were able to link the medallion up with Gideon and he took her to celebrate with champagne. As they ate, Rory accidentally revealed that she was a time aberration.
She angrily confronted her father and asked if it was true. Stein told her it was true and she felt horrified. Lily realized that this was the reason that her father had been distant during the Dominator attack; she was a complete stranger to him. She then became angry as she was sure that her father felt she was a mistake. Lily later asked Gideon to take her back to Central City but Gideon told her that her father gave orders that she wasn't to leave the ship. After talking with Jax, her father then approached her and told her that he was sorry for not telling her the truth. She said that it wasn't his fault that she was a mistake but her father said that he made many mistakes in his life but she wasn't one of them. He told her that time travel doesn't often give them what they want but it does tend to give people what they need and he needed Lily in his life in ways he couldn't have imagined. They shared a hug and Lily kissed her father goodbye before she returned to Central City.
Becoming a mother
Lily had been on sabbatical with her boyfriend in Rome and soon discovered she was pregnant. The both of them came back home to break the news to her family and her father was ecstatic at the news of becoming a grandfather. She was then reunited with Jax and played a small joke on him when he noticed she was pregnant. When Lily told Jax of how it was her boyfriend that was the father, her father told Jax that she had meant to say her future husband (as Stein didn't like the idea of Lily raising her child alone.)
Her father visited her using the Waverider's jump-ship to give Lily a new communicator to talk to her no matter what time he was in, so that he could be notified when the baby was coming. Lily contacted her father when she felt it was time and he came along with Jax and Rory to be there for the birth of her new son, Ronnie.
Barry and Iris' wedding
Lily went to Jitters to attend Barry and Iris's rehearsal dinner along with her mother. They were sitting with Jax when her father showed up a little late. Her mother told him that he was late as usual while Lily pointed out to her father that he had a time machine. He called out her joke as drool. Lily later attended the wedding, which was soon attacked by Nazis from Earth-X. She and her family were then taken to safety.
When Jax returned from Earth-X, he visited the Stein home to inform Lily and her mother of Martin's death. The three of them embraced each other in tears. At her father's funeral, Jax apologized to Lily as her father was looking forward to being with his family full-time. Lily assured Jax that a part of her father would always live through him.
When the Legion of Doom rewrote reality, Lily's existence continued in the new world. Her father had to work a lot under the employment of Eobard Thawne but he tried to take a day off to celebrate her birthday. However, both Eobard and an alternate Jax wouldn't allow Martin to leave to see her.
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. Martin mentioned that Lily once caused her kindergarten teacher distress when she insisting on singing the periodic table rather than the alphabet. 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. She also mentions to be bad at taking vacations, choosing to spend her time in Tahiti focusing on her scientific pursuits.
Lily was very close to those around her, sharing her parents' affections and care for Jefferson Jackson, often treating him in their time together like a brother. At her father's funeral, she and her mother embraced Jax, telling him that he would always be a part of their family. This shows that Lily is an incredibly empathetic and compassionate woman, forging familial bonds that are not limited by blood.
- Genius-level-intellect/Master scientist: Lily is an incredibly intelligent scientist, possibly exceeding even her father, with an expertise in nanotechnology and molecular biology.
DC's Legends of Tomorrow