Initially a student who has traveled to Egypt, Dio Brando sensed his Stand powers and asked him for a demonstration, which turned out to be a fight. Afterwards, Kakyoin is brainwashed by Dio using buds from his own cells into following his will, which is to kill Jotaro Kujo.
The manner in which he carries out his assault upon Jotaro differs from the Manga and OVA. In the manga (and the videogame), the assault happens inside a clinic of a school. In the OVA, he attacks Jotaro by possessing the body of a female student with his Stand. He is defeated by Jotaro either way.
Jotaro soon discovers that Kakyoin was under the influence of Dio. Jotaro then used his own Stand, Star Platinum, to remove Dio’s implant buds from Kakyoin’s head in spite of the risk that it may openly attack both of them. Kakyoin is freed from Dio’s slavery afterwards. He then joins Jotaro’s journey to save Holly and defeat Dio in Egypt.
His Stand is Hierophant Green, which can attack from long ranges and unleash an energy attack known as the Emerald Splash.
He maintains a calm demeanor and is often seen with Joutarou, consistently wearing his green school uniform. He’s often smiling with an “above-it-all” air, but humbles himself constantly and will go so far as the occasional crude joke to lighten the mood.
Kakyoin isolated himself from others at a young age, unable to trust people who couldn’t see his Stand. It is implied Dio was the first Stand user he ever encountered, a factor in his hesitation that led to him being implanted with a flesh bud.
He bonds to his compatriots during the journey, acting as an objective third party assistant. He plays a vital role during Polnareff’s battle against J. Geil, with fast acting, maintaining a calm thinker, and discerning the final plan of attack that would lead to his defeat.
He single-handedly defeats two Stand users, Tower of Gray and Death 13 (the latter of whom no one retained a memory of), and assists in the defeats of D’Arby the Player and Dio Brando.
He is killed by Dio during their fight in Cairo. Immediately before dying, he unleashes his final Emerald Splash to destroy a clock tower as a message to show Joseph that The World is able to stop time.