Ginjou Kuugo was a man who approached Ichigo after he lost his Soul Reaper powers, jumpstarting the plot of the Lost Agent of the Soul Society Arc. Ginjo was first encountered after being robbed, where Ichigo beat up the thief to retrieve his belongings. The only contents of the stolen bag that were shown was a Substitute Soul Reaper Badge.
Ginjou would spend the next few days visiting Ichigo and leaving cryptic clues such as "how much do you really know about your family?" and "I wouldn't trust Kisuke Urahara." Later, he claimed this was just to excite Ichigo's curiosity.
When Ichigo finally sought him out, Ginjou revealed his purpose: He was the leader of a group named Xcution, which consisted of FullBringers--humans who had powers, usually from being attacked prenatally by a Hollow--and this group sought to rid themselves of their powers, by giving them to a half-human-half-Soul Reaper. This would also restore Ichigo's powers.
As the training progressed, a former associate of Ginjou's named Tsukishima occasionally attacked the people around Ichigo. First Uryu, then Orihime, then later he attacked Xcution itself, particularly Chad. Just as Ichigo mastered FullBring, Tsukishima revealed his ability--to insert his presence into his target's past. This resulted in almost all of Ichigo's friends, family, and Xcution turning to Tsukishima's side. However, they still held loyalty to Ichigo--from their perspective, Ichigo was acting crazy and attacking their mutual friend who helped them through so many trials. They wanted to help Ichigo "apologize" to Tsukishima, which sent Ichigo into a homocidal rage, prompting him and Ginjou to attack Tsukishima directly.
During the battlle, Ginjou was hit by Tsukishima's attack, taking an attack meant for Ichigo. He claimed he was still okay for the time being. Soon afterwards, Uryu arrived, and he and Ichigo argued over whose side he was really on. Frustrated, Uryu exclaimed that he didn't understand why Ichigo wouldn't get away from the man who attacked him. Ginjou and Tsukishima used this moment to incapacitate Ichigo and Uryu, respectively. Ginjou then revealed that he had been working with Tsukishima previously, and had him use his Book of the End to make them "enemies."
Ginjou absorbed Ichigo's FullBring, but soon afterwards, representatives from the Soul Society showed up. They restored Ichigo's power with a device powered by their Reitsu and created by Urahara. With his new powers, Ichigo effortlessly defeats Ginjou, but he re-emerges, re-energized by Ichigo's FullBring. Captain Hitsugaya then reveals that he was the first Substitute Soul Reaper. Xcution and the Soul Reapers then split up into separate duels, with Ginjou battling Ichigo and Uryu.
The fight is relatively close, but with Ginjou having the apparent upper hand. He stops to explain that the Substitute Badge's real purpose is to moniter Ichigo in case he might be a danger to the Soul Society, and claims that he should be fighting then. Ichigo releases his Bankai, and explains that he more-or-less figured this out already, but did not mind. Ginjou then released his own Bankai to counter Ichigo's.
A short, but intense fight followed. When Ginjou couldn't overpower Ichigo, he tried firing a Cero at him, which Ichigo blocked with his hand. He then leapt forward, slicing Ginjou down his torso. They crashed back to earth, at which point Ichigo severed Ginjou's blade and struck the killing blow. Ginjou struggles to get back to his feet, before collapsing and wondering if things might have been different between them, under different circumstances.
Grief-stricken, Tsukishima attacks Ichigo. Riruka takes the attack for him, and berates him for using an attack meant to kill. Tsukishima yells desperately for her to move, and that they can't let Ginjou die. Riruka yells at him again, explaining that Ichigo saved Ginjou when they could not. Tsukishima cries over Ginjou's death, wondering what he's supposed to do now. Moe arrives to carry him to safety. En route, Tsukishima realizes that they are friends, and wonders if this is what Ginjou wanted him to realize all along. He dies soon afterwards.