When Jin was a little boy, he was very dependent on his older brother, Ragna, for affection and constantly pestered him to play with him. He saw his sister Saya as an obstacle of getting his brother's attention and constantly bullied her in retaliation. Eventually, Ragna decided to look after the sickly Saya and distanced himself from Jin, thus increasing his jealousy. When Jin was originally planning to kill her, he was then visited by his sister and being given the Nox Nyctores named Mucro Algesco: Yukianesa.
It was then Yūki Terumi entered Jin's life and told him about the Black Beast, that it would be born from Ragna, and that he is destined to destroy it. Deciding that his brother must pay for his ignorance, and to get rid of the obstacle between them, Saya, Jin left Terumi to do as he pleased, including burning the church where they stayed and murdering their caretaker. When Ragna arrived, he only watched in shock as Terumi cut off his arm, and kidnapped Saya. Jin later vanished from Ragna's life, and apparently his memory of that incident was blocked off (likely due to Terumi's ability to eat memories).
Jin was later adopted to the high-esteemed Kisaragi family. With his talents, he rose up in the ranks of the Novus Orbis Librarium, but his other siblings never trusted him, thinking that he only got his ranks due to his adoption. Indeed, Jin himself felt completely empty even after being showered with awards and ranks (which included the stopping of the Ikaruga Civil War).
When the rumors of the "Grim Reaper" arose, Jin felt a sudden urge to meet him, which caused him to abandon his post and head to Kagutsuchi to find out more about the Reaper.
When it turns out to be Ragna, Jin became ecstatic to meet his beloved brother and his psychosis caused him to be eager to kill him and fulfill his destiny.
During one of the time-loops, Ragna and Jin's battle is interrupted by Nu's appearance and she ends up throwing herself along with Ragna into the Cauldron. Never wanting to lose his brother again, Jin jumps into the Cauldron Jin Kisaragi,
despite his wounds and is sent back in time to the year AD 2099/12/31, where he meets Rachel Alucard and is offered a chance to be a true hero. From this point on, the Jin of this timeline became one of the Six Heroes, Hakumen
