Young Sheldon season 4 finale sets up Missy's arc in The Big Bang Theory. The CBS hit sitcom has just wrapped up its current year with its 18th episode, "The Wild and Woolly World of Nonlinear Dynamics." While the series had a significantly shorter run compared to previous seasons, much has happened to the Coopers, including Missy significantly growing up and starting to explore romantic relationships.
While Sheldon also dabbled with the idea of puppy love in Young Sheldon season 4 after he developed a crush on Paige, his twin sister is taking things much more quickly. At one point in the year, she went on her first movie date behind her parents' collective back but with Georgie serving as a chaperone. This was the only primary arc for Missy in the year as she mostly served as a supporting character as Sheldon took the spotlight. But this particular plot point led to the main storyline going into the Young Sheldon season 4 finale.
Missy suffered her first heartbreak in the outing as she reveals that her supposed boyfriend, Marcus, spent time with another girl. This understandably upset her and her bad mood kicked off a series of events in the Cooper household, including an intense argument between George and Mary. However, Sheldon's twin was unaware of the commotion in their household because she decided to run away. While Missy was understandably upset about what happened, her reaction in Young Sheldon seemed to be rather extreme considering the situation. Sheldon was eventually able to pacify her as he accompanied her to an unused cabin where they camped out for a while. This episode set up just how romantic relationships deeply affect Missy. She falls hard and fast, causing her actions to be dictated by her feelings, as evidenced by wanting to leave home. In The Big Bang Theory, Missy devoted herself to a marriage that didn't work. And because of that, unfortunately, there wasn't much else going on in her life, unlike her brothers, who were thriving at their respective careers.
Comparing Sheldon and Missy with each other is inevitable considering they're twins, and often, the former gets all the attention since he's unique in more ways than one. He's obviously intelligent, but that doesn't mean that Missy isn't, as well. In fact, all three Cooper kids turned out to be capable, with Georgie having the business acumen that resulted in him building Mr. Tire. Missy, on the other hand, is wise and street-smart; she's also more well-rounded compared to Sheldon. Her outburst at Mary in the Young Sheldon finale about how everyone always takes her brother's side was actually justified. It's a shame that she wasn't able to broaden her horizons, instead fully focusing on her marriage to the exclusion of everything else.
While The Big Bang Theory is finished, Young Sheldon can still continue revealing details about the future, including how Missy is. In its recently-finished season, the CBS prequel included a passing comment about how she's happy in her life but without any context or additional detail. Hopefully, moving forward, Missy's story continues to evolve and she becomes a fully-formed character in her own right.