The Joker appears after you defeat The Heart and is the final boss you face in Area 5. Defeating him grants you access to the Area 6.
It's discovered that The Joker was previously a member of the Kamikaze Union. After challenging The Emperor and losing miserably, he left the Kamikaze Union and was out for revenge against The Emperor. When Arata The Red was confronted by The Joker and his gang, The Emperor stepped in to help Arata. The Joker and his gang hospitalized The Emperor, stole his car and The Joker began to impersonate The Emperor. The Joker and his gang then began to terrorize many people under the name of The Emperor. He is also responsible for sending threatening letters to Alice The Mystical herself and Izumi The Popstar's group. After defeating The Joker, he grabs Namiko, pulls a knife out and threatens to kill her. Before he can do anything, Sho (The Emperor) jumps out from hiding and attacks The Joker, knocking The Joker out.
The Joker drives The Emperor's Jet Black Pearl Nissan GT-R R35