Red will become a playable character in Get Even only after you've completed all basic stages available to Black. Specifically, the game will switch to Red (Ramsey) after Black, controlled by the player, reaches the server room in the basement of the mental asylum and examines the large capsule, discovering something very important about himself.
Playing as Red, you'll be able to play through eight new memories, grouped in pairs - the first section will always be an investigation (often requiring you to deal with hostile characters), while the second one will focus on important plot-related discoveries.
Note! From this point on, you'll be controlling the character of Red. This doesn't mean, however, you'll be completely unable to play through memories as Black anymore. You can return to the main menu and choose the option to continue the game as Black or Red. Once you choose this option, you'll be taken back to the evidence room inside the protagonist's mind from where you can replay the memories (e.g. so you can act differently or find some missing evidence).
Take a moment to thoroughly investigate the basement. There's as much as eight documents scattered across the room. Other than that, notice the two monitors standing on the desk. The left screen is used to browse unlocked memories - the list will be empty at the moment, as you haven't completed any of them yet. The right screen is used to confirm your will to initialize a new memory - the first one on your list will be "Grace Kidnapping"