"Jolyne! I only came here to get you out!" —Jotaro Kujo P6
Talk to Jotaro at the tree, then do his quest. Once completed, talk to him again, and you will be awarded with Star Platinum: The World.
Jotaro Kujo: Hey, I'm Jotaro Kujo, a stand user. If you have a stand like mine, I can train you to evolve it.
Player Options: Ok. & Sorry, goodbye.
Jotaro Kujo (If you pressed Ok without a Mysterious Hat): Well, you're going to need a stand like mine, and a form of payment. I left something precious to me around here somewhere, maybe if you had that...
Player Options: Hmm. Okay.