Yumiko was a minor character in both the original Sailor Moon manga and anime. She hung around Usagi and Naru as a classmate at Juuban Municipal Junior High School, usually paired with her friend Kuri. Before the other Sailor Senshi showed up, the two of them basically filled out the cast—often filling viewers and readers in on what was happening around town, whether that meant new shops, events, or random people of interest.
In the manga, Yumiko was one of several girls who ended up buying cursed jewelry from Morga, Jadeite's Youma, at Osa-P. The anime gave her a bit more to do: she got hypnotized alongside Kuri by Derella in episode 7 and got swept up in the Cinderella Caravan talent search, and later popped up in episode 25 to tell Usagi about Crane Game Joe. Once the actual Senshi took center stage, though, she basically faded into the background.
Content compiled by AnimeList.moe from publicly available sources.
