The Simpsons uses a floating timeline in which the characters do not physically age; as such, Lisa is always depicted as eight years old. The show itself is perpetually set in the year of broadcast (except for occasional flashbacks and flashforwards). In several episodes, events have been linked to specific time periods, although this timeline has been contradicted in subsequent episodes. Lisa's year of birth is given in "Lisa's First Word" (season 4, 1992) as 1984, during the Summer Olympics. The episode "That '90s Show" (season 19, 2008), however, contradicts much of the established backstory; for example, it presents Homer and Marge as being childless in the late 1990s. Lisa is a lover of music, especially jazz. She enjoys playing the saxophone and became friends with jazz musician Bleeding Gums Murphy, whom she regards as an idol. Murphy helps pull Lisa out of her depression in "Moaning Lisa" (season 1, 1990). She is later deeply saddened by Murphy's death in "'Round Springfield" (season 6, 1995).
Lisa has been friendly with several boys, including Ralph Wiggum in "I Love Lisa" (season 4, 1993), Nelson Muntz in "Lisa's Date with Density" (season 8, 1996) andColin in The Simpsons Movie (2007). Bart's best friend Milhouse Van Houten has a crush on her, but despite dropping unsubtle hints about his feelings, he has been unsuccessful in winning her affection.
Lisa is the most intellectual member of the Simpson family, and many episodes of the series focus on her fighting for various causes. Lisa is often the focus of episodes with "a real moral or philosophical point," which according to former writer David S. Cohen is because "you really buy her as caring about it." Lisa's political convictions are generally liberal and she often contests other's views. She is a vegetarian, feminist, environmentalist and a supporter of gay rights, and the Free Tibet movement. In a special Christmas message for the UK in 2004 Lisa showed her support for Cornish nationalism, even speaking the Cornish language to get her message across. While supportive of the general ideals of the Christian church in which she was raised, Lisa became a practicing Buddhist in the episode "She of Little Faith" (season 13, 2001) after she learned about the Noble Eightfold Path.