Keiko Yoshii

Lain's homeroom teacher, is a minor character who appears in episode 1 and episode 8. She has very few lines.

Her name, Yoshii Keiko (吉井恵子), is only known because it appears in the credits.

Serial Experiments Lain @wiki mentions her on this page (Japanese).

Characters of the Serial Experiments Lain anime

Iwakura Family

Lain Iwakura • Miho Iwakura • Mika Iwakura • Yasuo Iwakura

School

Juri Katou • Alice Mizuki • Reika Yamamoto • Chisa Yomoda • Alice's Teacher • Lain's Teacher

Cyberia

JJ • Masayuki • Myu-Myu • Taro

Knights of the Eastern Calculus

Shoko Masatsugu • Masuoka Takuyoshi • Freeter

Conspiracy

Masami Eiri • Men in Black

Other

Cheshire Cat • Professor Hodgeson • Shou Masatsugu • Nezumi • S • Deus