One of the elements that's made The Simpsons so enduring is that it doesn't necessarily have a single lead character. While many episodes focus on Homer, the entire family has shared the spotlight over the last three decades of television. Many of their supporting casts have gotten their own episodes and storylines as well. In some cases, these supporting casts grow into fully fleshed out figures themselves, leading to initially bit-characters like Carl Carlson getting a great deal of depth.
While Homer, Bart, and (to a lesser extent) Marge's connections with the rest of Springfield have been fleshed out with the other residents of the town, Lisa's never really had much of a supporting cast to match them. While this has benefited the character herself and the overarching stories afforded her over the years, it also prevented any of the girls in her orbit from getting the same development that Bart and Homer's friends got. Here's why Lisa Simpson never got the kind of friendships that Bart has in The Simpsons -- and the impact it had on the show as a result.
Why Bart Got A Supporting Cast and Why Lisa Didn't
Within the world of The Simpsons, Bart and Lisa lead very different lives in Springfield. The prankish Bart fluctuates in terms of impact at Springfield Elementary. Sometimes he's among the more popular kids at school, and other times, he's a frequently bullied class clown. Still, he has a far better reputation among his peers than Lisa, who is often portrayed as a solitary and largely isolated little girl. Going all the way back to Season 1's "Moaning Lisa," Lisa's loneliness and inability to mask it has been an inherent element of the character. These divergent paths for the siblings resulted in a lot of differences. But one of the most impactful on the show as a whole has been the supporting casts that developed as a result.
Bart's friend Milhouse may technically pre-date The Simpsons, but his time as Bart's best friend led to a wealth of character development for him and his family. Characters like Nelson and Martin evolved into their own characters, starting out as side-characters in Bart's orbit before getting their own mixed amounts of development. Even fairly static figures in Bart's life -- like the bullies Jimbo, Kearny, and Dolph or the twins Sheri and Terri -- have gotten their own distinct personalities. By contrast, Lisa has a far smaller roster of consistent characters to bounce off of.
Ralph began as one of her classmates, but steadily became his own dim-witted (and occasionally very tragic) character. The closest thing Lisa has to a best friend is Janey, who is arguably one of the show's most underdeveloped side characters. In fact, Lisa's gotten more complex relationships with Bart's supporting cast than anyone in her own orbit -- especially Milhouse and Nelson, who Lisa's entertained romances with.
A Lack Of Friends May Have Helped Lisa In The Long-Run
In practice, Lisa doesn't really need much of a supporting cast. As one of the smartest people in a town that largely doesn't value intelligence, many episodes have highlighted Lisa's inherent loneliness; having her own Milhouse-like best friend would undercut that story thread. It plays into the show's overarching themes centered around Lisa and her attempts to find her place in the world. It's also a shame, as it means her potential supporting cast lacks the kind of development that's been afforded to Bart's. Even minor gag characters in Bart's class like the perpetually sick Wendall have been given more personality than Janey, a character who's been in the show since Season 1, and whose defining trait can be argued to be her "fair-weather" status as Lisa's friend.
Lisa has developed some short-lived friendships, as in Season 6's "Lisa's Rival" or Season 9's "Lard of the Dance" -- but those characters are often only really featured in a single episode before shifting into the background. Similar to Bart's surprisingly deep-bench of one-off love interests, Lisa's friendships often only last a single storyline. Some episodes have touched upon this though, like Season 33's "Girls Just Shauna Have Fun" introducing a fun dynamic between her and the older teen character Shauna -- showcasing a childish and enthusiastic but overwhelmed side to Lisa that's incredibly endearing. Despite this, Lisa often ends up alone again.
Lisa's character development might have been different if she'd been given a greater focus on the people in her orbit, and her loneliness might not have played such a core part of her story. As a result, she may have been a weaker character, as her decisions to double-down on social issues and discover her own place in society have become common storylines for the character (and may even explain why no one in-universe likes her too much). This enhanced focus has benefited the character, but it does mean the show is lacking in terms of supporting characters who are young girls. While the lack of friends made Lisa a better character overall, it's a shame she couldn't help set the stage for the kind of growth characters like Nelson have gone through. Even Janey could have gotten more depth to her role as a wishwashy figure in Lisa's life.