Barney and Lily have a unique friendship, which is for the most part platonic. The two of them met sometime in 2001, after Barney had met Ted and Marshall. They mostra più of a stereotypical brother-sister relationship with them making fun of each other and coming to the other for advice. Despite this, they have shown to have a small interest in each other sexually.
From Barney's perspective, he most likely sees Lily as his best confidant despite knowing that she can't keep secrets. Barney let her know that his childhood dream was to be a professional violinist. Also, Lily was the first person he let know that he developed romantic feelings for Robin. He took her consigli Barney and Lily in letto on how to better his relationship with Robin so she could see him in a different light. He treats her like one of the guys, such as when he tells her dirty jokes that are meant to be heard da guys. Barney has shown sexual attraction to Lily. When he first saw her, he called Lily "hot". Marshall also has noted that Barney has tried for a long time to see Lily's breasts. Despite this attraction, Barney would never act on it because he sees her and Marshall "perfect together", shown when he secretly went to San Francisco to get her to go back to Marshall and with his emotional breakdown to when he, Ted, and Robin believed that she and Marshall were getting a divorce.
From Lily's perspective, she most likely sees Barney as a surrogate brother. She is shown throughout the mostra reprimanding Barney, both physically and verbally, whenever he does something insulting to women that she doesn't approve of. An ongoing catchphrase in the mostra is that whenever Barney does something that truly upsets Lily, such as blackmailing her, she always says, "You son of a bitch!". Lily has shown a particular interest In Barney's romantic life. Despite the other members of the gang believing that Barney is perfectly happy being single, she believes that deep down Barney does want to get married and start a family someday. She is always excited when Barney shows legitimate feelings for women, as seen with Robin and later Nora. Whenever this happens, she pushes him to pursue a committed relationship. Regardless of their sibling relationship, Lily seems to have an interest in his sexual activities. She expresses a bizarre obsession with Barney's sex life, shown when she asked Robin how he was in letto and if he liked using sex toys after sleeping with him for the first time.