Myu-Myu ミューミュー (Myūmyū)
Myu-Myu is a ten-year-old girl in Taro's trio. She is drawn to Taro because he seems grown-up and looks down on Masayuki as a kid. Japanese sources and the original booklet notes describe her as far more vain and catty than Lain, and quick to sulk when Taro pays attention to anyone else. She is jealous when Taro asks Lain out, though Taro mistakenly thinks she is jealous of Lain the other way round. The PlayStation game's start screen carries a line thanking 'myu-myu', a small in-joke between the two halves of the project. Yuki Yamamoto voices her in Japanese and Sandy Fox in English.
More (spoilers)
As Taro grows obsessed with Lain, Myu-Myu turns cold on him and tries to get him away from Cyberia. After the reset, the three children are friends again, crossing the street playing rock-paper-scissors, and she bosses Masayuki around as usual. Taro's NAVI then receives a mail about KIDS technology, a small hint that the experiments that used children like her have not entirely vanished from the new world.
Sources: en.wikipedia.org archived · ja.wikipedia.org archived · animenewsnetwork.com archived · lain.wiki · lain.wiki