Liam was a beast born in the 1800s to the Ellingsworth family. However, he was revealed to be the product of an affair his mother had "with a monster." The Ellingsworths left him to grow up in an orphanage, abandoning him after they found out the truth. Liam was engaged to Rebecca Reynolds, until she met and fell in love with another beast named Alistair. Rebecca left Liam for Alistair by 1854. Furious over her rejection, Liam became unhinged and obsessed, intent on ruining their relationship. He framed Alistair for several murders around New York. The people of the city believed Alistair was responsible, and killed him for his believed crimes. After Alistair died, Liam kidnapped and murdered Rebecca, as well.
Liam was first mentioned by Julianna Keaton, who Vincent Keller and Catherine Chandler found out was experimenting on innocent people to make them superhuman. Julianna, though, said she was trying to stop Liam, and says he tricked her into making him superhuman, then tried to kill her. Julianna escaped, and she has been trying to find someone who can stop him, and realizes Vincent is the only one who can. They don't believe her, though, having no proof that Liam exists.
Julianna tried to inject Vincent with the superhuman serum she injected into Liam, but was stopped before she could. At Vincent & Catherine's wedding ceremony, Tess Vargas, Catherine's maid of honor, shares how the NYPD found someone who fits the profile of Liam that Julianna described, and that he just came to New York. Liam is then seen standing atop the New York sky line, and is able to find Julianna inside the church from several miles away, using superhuman vision. He goes to the church, and rips Julianna apart, killing her. When Vincent and Catherine find Julianna's body, Liam attacks both of them before escaping, but they survive.
Liam sends a group of mercenaries after Julianna's experiments, including Tyler Zane and Alton Finn. Vincent tracks down a mercenary, and saves Alton before he can be killed. However, the mercenaries then go after J.T. Forbes, Catherine & Vincent's friend, who was inadvertently injected with the serum, but later cured himself. Before they can kill J.T., Vincent, Catherine, & Tess save him. Alton is placed under police protection, but Liam sneaks into the hospital room next door to Alton's, and places a bomb on the patient's oxygen tank. The explosion kills Alton and other people. Meanwhile, Catherine finds Russell Keaton, Julianna's brother, who is also being targeted by Liam. Russell reveals he injected himself with the superhuman serum Julianna created to stop Liam. Liam, though, finds him and kills Russell first.
Liam goes after J.T., and Catherine & Vincent follow him. Vincent tries to hold off Liam, and reveals himself as a beast, as well. Liam is surprised by this, and transforms, revealing himself to Vincent, Catherine, J.T., and Tess. Though he gets the upper hand and knocks down Vincent, Liam leaves without killing any of them, ominously saying how "this changes everything."
Liam approaches Catherine in public while she's with her sister, Heather. He lets Catherine arrest him, hoping to get to her and Vincent. At the police station, Liam says he needs Vincent's help with a task while throwing some veiled threats towards Catherine and J.T. When a lawyer comes in and gets Liam released, Vincent & Catherine find out the Ellingsworths were the ones who helped Liam.
Liam later shows up at Vincent & Catherine's place after kidnapping J.T. However, he lets J.T. go, because he's really after Vincent. Liam tells them that DHS has a sample of Alton's blood, as well as Liam & Vincent's, which risks exposing the two of them. Vincent agrees to help Liam, trying to find out what his ulterior motive is. Catherine approaches Helen Ellingsworth to get her to stop protecting Liam. She finds out, though, that Helen believes Liam is her brother. While Liam & Vincent break into DHS headquarters, Catherine has J.T. analyze Liam & Helen's DNA test, and both he & Heather figure out who Liam really is.
At DHS, Vincent realizes that Liam tricked him because he's trying to separate him & Catherine. Liam threatens to kill Vincent, but Vincent manages to get the upper hand and escape from him. Liam then sets out to destroy Catherine & Vincent, getting both of them in trouble at work. He poisons Helen, and later frames Vincent for her murder, making it look like medical malpractice. At Helen's funeral, Vincent tranquilizes Liam, and he's unable to transform. Catherine reveals herself holding a gem used to control beasts; Liam recognizes it, and Catherine reveals that Rebecca is her ancestor.
Catherine & Vincent try to stop Liam by erasing his memory, with the help of Catherine's father, Robert Reynolds. While he's drugged, they find out that Liam killed Helen in order to gather the whole Ellingsworth family, and kill them as revenge for abandoning him as a boy. Catherine & Vincent run to the funeral to save them, while Reynolds stays back with Heather & J.T., keeping a watch on Liam. The drugs, though, make Liam think Catherine is Rebecca, and he becomes intent on killing Catherine & Vincent the way he killed Rebecca & Alistair. He breaks free, and kills Reynolds before escaping.
Liam digs up the body of Curt Windsor, a beast who Vincent killed, and he kills the groundskeeper. Vincent is lured to the same spot by Liam, and framed for the crimes. Vincent tells Catherine that they should tell DHS the truth about Vincent being a beast so they will help to stop Liam. As they reveal this, Liam attacks the DHS agents, killing Director Hernandez. Vincent ends up running from the police, and falling off the hospital roof. Liam is near the building, making sure Vincent is dead. He hears Vincent's heart stop and leaves, satisfied.
Liam later comes after Catherine, the way he came after Rebecca. When Heather reveals herself, Catherine gives herself up to protect her sister. Liam kidnaps her, and brings Catherine to his home, under the streets of New York in the tunnels. However, Vincent, who faked his death, tracks Liam & Catherine. Vincent punches a hole through Liam's chest, killing Liam and saving Catherine.