Lament of Innocence is one of those games that comes with a friendly language select option, allowing players to experience both the Japanese and English voices in the cutscenes. As such, Walter has two voices that are easily accessible to the players.
Walter's Japanese voice is provided by Kishino Yukimasa. It seems like he's done more work on anime than on games, and is part of a theater group. A small selection of other roles is listed below. Visit the Anime News Network for more information.
Interesting tidbit: Marfisa says that the way Walter speaks is supposed to represent some sort of accent, but she's not familiar enough with Japanese accents to know which one. She thinks he sounds just like "the creepy Nazi-ish guy in the Meine Liebe anime" and has a very smarmy tone. Maybe it's a Japanese approximation of a German accent? He sounds like an oily car salesman to me.
Cowboy Bebop - Jonathan
Magic Knight Rayearth - Lafarga
One Piece - Higuma
Slam Dunk - Jun Uozumi
Trigun - Loose Ruth
Dead or Alive 3 - Brad Wong
Dynasty Warriors 3 - Cao Cao
Dynasty Warriors 4 - Cao Cao and Zhang Bao
Langrisser V: The End Of Legend - General Aizel
Walter's English voice actor is Jamieson Price, who has worked on a ton of anime and games. A small selection of notable roles is listed below. Visit Wikipedia for more information.
Bleach - Chad
Gankutsuou - Count
Rurouni Kenshin - Usui
Tenjho Tenge - Bob Makihara
Witch Hunter Robin - Takuma Zaizen
Dirge of Cerberus: Final Fantasy VII - Reeve Tuesti
Disgaea: Hour of Darkness - Seraph Lamington
Dynasty Warriors series - Lu Bu
Eternal Sonata - Legato
Shadow Hearts: Covenant - Masaji Kato
Suikoden V - Georg Prime