SF

SF (short for Sex Friends) is a Japanese erotic visual novel starring Youta Onose, an introverted virgin who is about to spend most of his summer alone in his room. One of his only friends is Rio Yoshizaki, his next-door neighbor and childhood friend who occasionally visits. Youta has had feelings for Rio since childhood, but had his hopes crushed when he saw Rio with another boy. When Youta confronts her about this, Rio explains that the boy Youta saw her with wasn't her boyfriend, just a "sex friend", and invites Youta to become one too in order to coax him out of his shell. Aghast at Rio's promiscuity, Youta must decide if he can accept this new side to her and evolve their friendship into something else.

Unlike typical visual novels which feature static backgrounds and character sprites, SF features only fully-animated video clips above the text box which contains the text of the story and the protagonist's inner thoughts. Rio's dialogue is performed by a voice actress. At certain points throughout the game, the player can choose to replay parts of a scene before moving onto the next scene. There is only one choice in the game that the player can make, which determines which ending the player gets.