Ellen McLain is the voice of Portal and the Overwatch. IGN explain to her who the Overwatch is.
"One of the many features that make Valve games stand out is the stellar voice acting. As you make your way through The Orange Box, there is one actor whose voice you hear in every game: the delightful Ellen McLain. She first appeared in Half-Life 2 as the voice of the Overwatch, the authoritarian government. Her voice is also heard during matches in Team Fortress 2 as the announcer. But she takes a starring role as the computer program GLaDOS in Portal, one of the most engaging characters ever to appear in a videogame". -IGN