Rainbow

"So... this is is the power of a "Stand User", what a fearsome..."

- Rainbow, after being defeated by the player

In-Game Role
Rainbow is the first optional boss of the game. (And probably the first boss in general, given it is highly recommended to fight her first).

A travelling fortuneteller, she can be encountered inside of the Game Center in Japan, after obtaining an eerie tarot card from battling the delinquents in town.

Not much is known of her history or her motivations (One may assume she works as a Slave of Fate), however, it is implied that she seems to hold a big grudge against stand users in general, attacking the player at the first sign that he/she gives of being one.

Ironically, after beating her the player is met with a short explanation of how she was hospitalized after her battle with you, unaware of her own capabilities of controlling stands.

Upon her defeat, Delinquents will stop appearing as enemies in Japan.

Chaos Mode
In Chaos Mode, Rainbow is an optional party member. After defeating her at the Game Center, she can be found outside of the hospital in Japan. There, she will tell the player that she wants to go back to Egypt, but can't because she lacks the money. Here, the player can ask her to accompany them as they travel to Egypt, or alternatively, pay the money needed to get her a plane ticket. If the player asks Rainbow to join them, she will enter the player's party and stay there until you leave Japan. While Rainbow is in the player's party, you are unable to enter your house, so it's important to finish anything you need to do there before you recruit Rainbow.