Peter, known as Your Boyfriend, is the titular main antagonist of Your Boyfriend. Peter is indeed a yandere who is completely obsessed with his love for Y/N AKA the player.
Background[]
On Day 1, Peter appears in front of the player when Y/N is sitting on the bench. He introduces himself and offers Y/N out to dinner. This is when the player decides which path they want to take with the story line.
Both Y/N and Peter agree to go out together for dinner. The dinner takes place at Y/N's work place. Y/N waited, meanwhile Peter goes out to buy a rose for Y/N. He gives the rose to Y/N as a gift. If all goes well, the date will be successful and both Peter and Y/N would be willing to see each other again.