Leonard Peabody's real name is Harold Jenkins. He was born on the same day as the Umbrella Academy siblings, but only by coincidence as his birth was a completely normal birth. His mother died during childbirth, causing his father to become an abusive alcoholic. Because of this, Leonard ran away to try and join the Umbrella Academy, but was turned away by Sir Reginald Hargreeves, and laughed at by the crowd. Later, his father made fun of him for his outfit and sent Leonard to fetch his beer twice. When Leonard took too long, his father would physically hurt him. Leonard was fed up with being pushed around, so he resorted to murdering his father with a hammer. He was sent to prison for 12 years due to the crime.
In episode 6, The Day That Wasn't, Leonard's name was found on an order that was meant to be given to Cha Cha at the commission by Five, as the person responsible for the apocalypse, which read "Protect Harold Jenkins." When Five came back, Allison researched the name and found out that Leonard's real name was Harold, validating her initial concern about him.
Leonard and Vanya go to his grandmother's house to convince Vanya to use her powers. Allison comes to find Vanya and tells her the truth about Leonard, but Vanya is upset about Allison getting involved with Vanya's love life. When Allison feels that she needs to resort to using her power to stop Vanya's rage, Vanya ends up nearly killing Allison, by slitting her throat with her violin bow. They run away, and he urges her to get revenge on her family. When Vanya refuses, he begins to mock her, constantly hitting a book and calling her ordinary and weak, resulting in her using her powers to kill him violently.
It is implied he murdered the first chair violin, as seen at the end of episode 5 when we see Helen Cho's body wrapped in a plastic tarp in Leonard's attic, in order for Vanya to get it, a theory that is given further evidence when he states he's the reason she got first chair. It is also shown that he murdered the last of the drunk thugs in order to avoid anyone finding out his involvement and intentions in hiring them.
Vanya Hargreeves Edit
Leonard had a romantic relationship with Vanya Hargreeves after becoming her violin student. To Vanya, Leonard was the only person who truly cared and loved her for who she was. He seemingly understood what she was going through, having experienced what it was like to be excluded and mocked for being ordinary. However, he never truly cared for Vanya and was only using her for her powers to get back at the Umbrella Academy. When Vanya discovers his true nature, he goes as far as to verbally abuse and belittle her. When Vanya retaliates by using her powers and murdering him, he seems surprised because he believed that Vanya still cared for him and wouldn't harm him.