TV-Series
Description
Juli "J" Sawaragi, also known as Jack Knight and Number Eleven during his time with the Thousand Spirits Group, stands as a central figure in the narrative. The son of Finnish electronics magnate S. Kiano and Japanese mother Miyako, he has a younger twin sister, Kyouka Sawaragi. Their considerable wealth affords the family residences in Kyoto, the Tokyo area, and a castle in Finland.

Initially celebrated as the "Noble Youth of the Battle Spirits World," J enjoys fame and a significant fanbase. His personality is cold and aloof, though he is deeply protective of his sister. A central conflict stems from his strained relationship with his parents; his mother disapproves of his singular focus on Battle Spirits, while his desire for acknowledgment from his father—a member of the antagonistic Thousand Spirits Group—drives him to temporarily join the organization under the alias Jack Knight. He believes strength and victory will earn his father's respect.

His primary motivation is a desperate need for his distant father's approval. This leads to a temporary Face–Heel Turn where he denies his family and friends to become Number Eleven in the Numbers Elite. During this period, he adopts an evil persona, wearing a black cape, and his deck temporarily incorporates red cards, featuring The DarkDragonEmperor Siegfried as a key card.

A pivotal development occurs when he is forced into a tag battle partnership with his father during a team tournament. Though successful in matches, J refuses to continue, culminating in a decisive battle against his rival, Bashin. This confrontation results in a Heel–Face Turn, with J resigning from the Thousand Spirits Group and reconciling with his father, who also quits the organization. Their conflict resolved, J promises to spend more time with his family.

Upon his return, he officially joins Bashin's team, Team Shomen Toppa. In the final King Uchuuchoten Cup tournament, he partners with Bashin in a tag match against the brainwashed Numbers Elite, Smile and Seven. After their victory, J assists in sabotaging the antagonist's systems, which leads to the loss of his white pyroxene stone and his pet cat Okyou's ability to speak. In the epilogue, he chooses to study abroad at Elliott's Battle Spirits University in America, though the stones' powers appear restored, allowing for a final battle with Bashin.

In the manga adaptation, his initial dislike for Bashin is more pronounced. His motivation for accepting power from Smile to join his side is explicitly to gain strength and defeat Bashin, thereby earning his father's notice. His defeat by Bashin in battle leads to his reconciliation with the protagonist, and he cooperates with the group to defeat Smile.

J is a strategic battler who prefers white attribute cards, utilizing a defensive and steady playing style focused on tempo control. His deck includes key cards such as The ImpregnableFortress Odin, The Gigantic Thor, The DarkDragonEmperor Siegfried, and The SacredEmperor Siegfrieden.

His appearance is characterized by short white hair, blue-green eyes, and a distinctive white bell-shaped cape. As Jack Knight, his outfit is identical but colored black.