Chisa Yomoda 四方田 千砂 (Yomoda Chisa)
Chisa is a girl from another class whose suicide opens the series: she jumps from the roof of a building in a Shibuya entertainment district. She was socially awkward and more comfortable in the Wired; Alice and her friends once took her to Cyberia, which she did not enjoy, and she once walked home with Lain. After her death, students receive emails from her saying she has only abandoned her body, that she is still in the Wired and that God is there. Konaka built the opening as a straight horror premise, a message from a dead girl, before turning it into something else. Ueda told FRUiTS that the suicide scene was not central to the plot but was meant to make viewers feel the value of life, and that TV Tokyo had to approve it in advance. Sumi Muto voices her in Japanese.
More (spoilers)
Chisa keeps appearing to Lain, at a railway crossing and elsewhere, smiling but never explaining where she really is. Ueda said at Otakon 2000 that when Chisa's lips move without sound, she is already dead and only the idea of her words comes through, shown as on-screen text. Eiri later implies that her death is tied to his promise that the flesh is unnecessary.
Sources: en.wikipedia.org archived · ja.wikipedia.org archived · animenewsnetwork.com archived · lawmune.net · lawmune.net · konaka.com archived · lain.wiki