Anyone who has watched at least one full season of Young Sheldon knows how little the famous Big Bang Theory prequel actually focuses on Sheldon himself, and how much attention the writers pay to the other members of his family. This is what makes the show so much more emotional and appealing to a wider audience.
However, for those who haven't seen Young Sheldon, there is one character whose entire existence feels highly unnecessary. Missy Cooper could have been erased from The Big Bang Theory universe and absolutely nothing would have changed, so why was she brought up in the first place?
Sheldon's twin sister, who is supposed to be his complete opposite, was mentioned several times on The Big Bang Theory before the character was reintroduced in Young Sheldon. She appeared physically twice, in season 1 episode 15 "The Porkchop Indeterminacy" and ten years later at her brother's wedding.
In the first season, her character was used almost as a plot device to make the characters compete for her attention. There was apparently no intention of introducing her into Sheldon's life since there was no place for her in it. Her appearance at Sheldon and Amy's wedding was nothing more than a nostalgic nod.
Since there was no lore behind the character and no apparent need to include her at all, many fans wondered why she was even brought into the series.
What really stands out in the show is Sheldon's ability to form a genuine relationship with someone who is in no way like him. This side of him is explored with Penny, who is basically the same character as Missy, but lives closer and is not related to him at all.
For Sheldon to have a closer and more trusting relationship with Penny than with his own sister of the same character and values is almost a huge disservice to her character.
The only way Missy Copper could have been improved is through the prequel, and Chuck Lorre must have known that all along. Otherwise, it's almost impossible to find a reason for Missy to exist.