It's the Window_Battlestatus class.
Mind you, the Battlestatus window takes up the bottom 120px of the 640x480 area.... and viewport 1 for the enemy battlers and battle background in the top is set to 640x320 and not 640x480. If you want the Battlestatus window to be transparent or semitransparent with a full-area 640x480 battle background, you need to set the Spriteset_Battle's viewport1 to accommodate.