Isaac Stone

Isaac Stone is a character in Xenogears. He is a bishop of the Ethos organization.

Story arc
Stone is among the leadership of the Ethos, holding the title of Bishop. He acts as a role model to Billy, saving him from an attack by Wels and inspiring him to work for the Ethos. Shortly after Billy's introduction in the game, he asks Billy to assist with the investigation of a Reaper ship that has appeared in the area.

In actuality, Stone has been manipulating Billy over many years as part of an elaborate revenge against Billy's father, Jessie. Stone is originally from Solaris, and his real name is Stein. A fellow student of Jessie's in Jugend, he fell in love with Racquel, who instead married Jessie and was the mother of Billy and Primera. Stein also competed with Jessie for the command of Gebler and was beat out by him shortly before Jessie fled Solaris entirely. Stein would eventually head to the surface under a false name and after finding Jessie's family arranged to have Racquel killed by Wels. He then "saved" Billy from the Wels, tricking him into being loyal to him.

In the present day, Stone leads the Etones, a team of assassins who are responsible for assassinating corrupt Ethos priests and officials who seek to break off from Solaris. After returning from the Reaper Ship, Billy finds most of the Ethos members assassinated, and Stone appears to him, revealing the truth about his role with the Ethos. He flees the Ethos Headquarters on the Gear Alkanshel and heads to nearby Zeboim ruins in order to recover the nanomachine colony Emeralda, which he succeeds at through the help of the Elements Tolone and Seraphita.

After bringing Emeralda to Krelian, Stone is fused with the Alkanshel, resulting in all the skin being removed from his face. He attacks the Yggdrasil and is further empowered by Grahf. At this point, he reveals the entirety of his past relationship to Billy, as well as the fact that the Wels Billy has been killing were originally humans. However, through the assistance of Jessie and a special Gear weapon, Billy and the others are able to defeat the Alkanshel, killing him.