Kūgo
Ginjō

Kūgo Ginjō was the first Substitute Shinigami, appointed long before Ichigo Kurosaki and issued the same badge. What nobody told him is what the badge actually is: a tracking device and a leash, handed out by Soul Society so a human with borrowed power can be monitored and disposed of when he becomes inconvenient. Ginjō found out on his own, and the discovery is the origin of everything he does afterwards.

He built Xcution out of Fullbringers with nowhere else to go and spent years planning to take Ichigo's powers for himself. The training he puts Ichigo through is real, the endurance work is necessary, and the encouragement is entirely sincere in its mechanics and entirely false in its purpose. Ichigo gets his strength back and Ginjō takes it the moment it is complete.

His Fullbring is a broadsword drawn from his own Substitute badge, and after copying Ichigo he can produce a Hollowfied version of it he insists on calling a Bankai. The fight that ends the arc is Ichigo against a man whose grievance is legitimate and whose method is not. Ginjō dies without ever being told he was wrong about Soul Society, because he was not, and Ichigo buries him in the Human World rather than let the Gotei 13 take the body.