Darla Shannon is Jessie Prescott's old High School rival and the main antagonist in the 2-part episode "G.I. Jessie".
In the episode, Darla notices Jessie and recalls the time that Jessie fell down a well and laughed on how she was never successful. She acts shocked that their parents were gonna marry each other. Darla stated that there was a feud that happened between them since the 1920's and that she always wanted it to remain intact, even though it had absolutely nothing to do with herself.
Throughout most of the episode, Darla tried to keep her brother from dating Emma Ross, even though she treats him badly (most likely due to Emma being relevant to Jessie). It was implied that she made him smell her armpit. The same goes for her dog from dating Mrs. Kipling. After the wedding, she fell down and had cake all over her. In the end, she stood around near Jessie, feeling hateful and ignored.