Masayuki マサユキ
Masayuki is Taro's eleven-year-old best friend and the tallest of the three children. He knows NAVIs well but less than Taro, and he usually lets Taro lead. He is the one who explains to Lain how to install the Psyche processor. Japanese notes describe him as tolerant and steady: he shrugs off Myu-Myu calling him a kid, notices how Taro really feels, and even wakes a sleepy Taro when Lain walks in. He once suggests buying the Psyche from Lain, which Taro vetoes. Sora Fujima voices him in Japanese and Dorothy Elias-Fahn (as Dorothy Melendrez) in English. He is the quiet straight man of the trio and rarely gets a scene to himself.
More (spoilers)
Masayuki has no big turn; he quietly watches the trio fall apart as Taro fixates on Lain and Myu-Myu loses patience. After the reset he is back with the other two, losing at rock-paper-scissors and, as Japanese notes point out, stuck carrying the bags. In the same scene the boy who died at Cyberia under Accela walks past with his little sister, alive again, and admires Taro's modified machine.
Sources: en.wikipedia.org archived · ja.wikipedia.org archived · animenewsnetwork.com archived · lain.wiki · lain.wiki