Jarden is an original character created for Legendary Guardian and has no basis in cannon from Square Soft in Final Fantasy X. He is mentioned in two chapters of LG.

Part of a group of four Summoners that accompany Auron’s uncle, Youke, to the Calm Lands; Jarden and his Guardian Takk Ronso become fast friends with the Second of Bevelle. After Jarden mysteriously obtains a new Aeon, the tide of battle is turned and Youke helps the Summoner and Guardian lead the vanguard party.

Later, Youke mentions Jarden to his own Summoner – the Maester Umaro. As the two prepare to ascend Mt. Gagazet, Auron’s uncle mentions that Jarden fell to fiends before reaching the mountain summit. His Guardian, Takk, survived the fateful trek.

Jarden was, in my mind, a priest who studied the Summoner’s art in Bevelle after spending time at the Gagazet shrine. My mental image of him is based on a FFX cannon character, Isaaru. Long brown hair worn in a ponytail, brown eyes, and a face with Asian features is a good summary of my image of him. Surely, he must have been a very honorable and focused person to have commanded the respect of a Ronso guardian and Youke, the Second of Bevelle.