Early in Stardust Crusaders, the third arc of JoJo's Bizarre Adventure, the hero Jotaro Kujo has a run-in with an enemy Stand user going by the name Noriaki Kakyoin. Jotaro is fresh out of jail, and he is still trying to come to terms with his Stand, Star Platinum. All Jotaro wants is to resume his ordinary life, but he's about to get dragged into his first major Stand battle. Fortunately, there is a chance to turn his enemy's life around.
Battle In The Nurse's Office
Jotaro's first run-in with Kakyoin was a subtle one. Using an enchanted paint brush, Kakyoin drew a picture of Jotaro and marked Jotaro's ankle with a red line, causing the real Jotaro to fall down a long flight of stairs. Kakyoin then introduced himself, but didn't give himself away as the perpetrator. Later in school, Jotaro ended up in the nurse's office with two other students, and Jotaro discovered a threatening note from a certain Noriaki Kakyoin, promising to attack with his Stand.
Kakyoin soon made good on that threat, using his Stand Hierophant Green's elastic nature to send it into the nurse's body and possess her. Hierophant Green gave the nurse superhuman strength, and she went on a rampage with a pen until Jotaro restrained her and used his own Stand to drew out Green Hierophant. Now that Kakyoin didn't have the innocent nurse as a hostage, Jotaro was able to fight Kakyoin directly, only to take a serious hit from the Emerald Splash technique. Star Platinum wasn't done yet; it survived the next attack and returned the favor, beating Hierophant Green (and by extension, Kakyoin) senseless. Kakyoin was out cold, and Jotaro knew that Joseph Joestar should look him over and make an assessment.
DIO's Secret
Jotaro is a gruff kid, but he is not the type to just kill anyone who gets in his way. He's curious to know more about DIO, and he's sure that Kakyoin may serve as a solid lead. So, he brings Kakyoin's unconscious form back with him to the Joestar household, and lays Kakyoin down on the tatami mats in a spare room. Star Platinum can feel the inside of Kakyoin's head, and Jotaro realizes that a parasitic flesh bud is in there. No doubt it's forcing Kakyoin to act the way he is.
Avdol and Joseph aren't so sure that the flesh bud can be safely removed. If Jotaro slips even slightly, he will accidentally kill Kakyoin in the process, but Jotaro is determined. His Stand as remarkable precision as well as speed and strength, so he dives right into Kakyoin's head. It's not easy, but Jotaro manages to wrench the flesh bud out of Kakyoin's body, taking care not to damage his brain with the flesh bud's tendrils. Star Platinum ties up the flesh bud into a knot, and Kakyoin is free.
Kakyoin is himself once again, and he has zero interest in fighting on DIO's behalf any longer. He's outraged when DIO's influence envelops Holly Joestar in a plant-like Stand that is slowly killing her, and Kakyoin is determined to save her life. So, he joins Jotaro, Joseph and Avdol on the mission to trek to Egypt and slay DIO once and for all. Kakyoin now fights for the innocent, not for DIO, and he won't let anyone control him ever again.