I haven't found a heart either... I thought the knife is for that and even tried it on the girl but it still results in just a skin, even after being gifted 10 flowers.
How does one get a heart in this game?
To get a heart you must gkill the girl when she is at the bench. Ive seen a video of it, but the guy used savestates, wich makes the timeline too complicated to understand. You talk to her, and i guess you do some other stuff every day, untill she asks you to enjoy the sunset on the mountain, and you kill here there, without going on the bench, as that end the game.