Jin Kariya's original name is Eugene Currier, but he adopts a Japanese-style name later in his life. He has a scar on his chin caused by a slash from a hollow when he was young, short white hair, long upward-curving eyebrows and bright red eyes. He is the most powerful member of the Bount group. In a flashback, it is revealed that Kariya had brown hair and brown eyes when he was a young boy. His hair color and eye color changed when he first merged with his doll.

When first introduced, Kariya's goal was to acquire massive numbers of living spirits to increase the power of himself and his followers, most notably through the summoning of the bitto. The techniques used had a 50% chance of ripping open the barrier between the mortal world and Hueco Mundo.

This initial plan, however, has been revealed as merely a means to an end. His ultimate goal is not simply to increase his power, but rather to invade and overthrow Soul Society. To accomplish that, Kariya has obtained a device (Jōkaishō) from the Shinigami Research Institute which he claimed would allow him to obliterate Seireitei. However this could also be a means to end his eternal existence.

Kariya seems to be very knowledgeable and intelligent as he has great knowledge of Soul Society. He knew who Byakuya Kuchiki, Rukia Kuchiki and Ichigo Kurosaki were before even meeting them. He was aware of the Four Great Noble Houses of Soul Society, and knew of existence of the Jōkaishō in the Shinigami Research Institute in Seireitei. He is also able to speak both German and Japanese.

Kariya is revealed to have no emotional attachment to any of his subordinates, as he mentions that he sees them only as pawns to achieve his own ends.

Kariya was first created by an experimental error which occurred in Seireitei. Later, in the Bounts' Cave in the real world, he was saved by Rantao from the shinigami who were trying to eliminate the Bounts. Right before Rantao told him to run away while she deals with the shinigami, Rantao gave Kariya his Doll, Messer ("knife", in German). Kariya merged with the doll when fighting Hollows, and since then his attitude changed, and he became determined to destroy Soul Society. Later, Kariya was against proposals to use the Quincy to return to Soul Society to live, thinking the shinigami would use the Bount as servants. He witnessed a war between the Bount and the Quincy, where the shinigami from Soul Society suddenly came and started to massacre the Bount. Kariya was deeply angered by this, and ever since then, he held a strong grudge against the shinigami. He also decided to give up on weak Bounts, forming his plan to gather all the remaining strong Bounts to him to destroy Soul Society. He perfected doll summoning from being hit-or-miss to a method that would sort out the strong Bounts from the weak, with failure resulting in death. In that way, the Bounts that were power-hungry stayed with Kariya. Sometime in his life, he learned that the shinigami were the ones who created the Bount. Knowing that the shinigami created his race just to destroy them, his grudge against the shinigami only became worse. He later married another Bount, Yoshino Sōma, after stopping her from committing suicide, though he never truly loved her. He was merely trying to gather the Bounts from all over the world in order for him to be able to achieve his goal of annihilating the Seireitei.

Kariya was close friends with fellow Bount Koga Gō for a long time. At some point, Kariya asked Koga to take care of a young Bount, Cain for him. Koga ended up failing when Cain was killed by his doll. At another point in time, Kariya and Koga saved a shinigami, Maki Ichinose, from a hollow in a desert. Ever since then, Maki became loyal to Kariya.