Gantetsu

is a former Shinobazu, or criminal ninja.

Background
While he was a member of the group he began to feel guilty for what he was doing. Gantetsu thought he was saving the orphans, but he still knew that losing their parents was something awful and decided to put a stop to the ninja dropouts. He saved the children from the houses they were raised in, hid them in the Forest of Bewilderment. He also stole the treasure they had stolen together, and then he let his comrades to believe that he was captured so that they would come "rescue" him in order to learn where the treasure. He hoped that by doing so his former comrades would get caught and that would be the end of the ninja dropouts. Despite this, he knows that he can never completely atone for his crimes.

Shinobazu Arc
Gantetsu was captured and was to be taken to a local capital city. On the trip down a river, his comrades, Shura and Monju, attacked a prison ship; while it was initially thought that they had been planning to rescue him, they were looking for him in order to find the money that he had hidden away. Although Todoroki did not trust him, and tried to kill him on more than one occasion, Gantetsu repeatedly helped the group navigate the forest and defeat the Shinobazu members, even saving Todoroki's life. Todoroki agreed to falsely report that he had died in order to allow him to remain with his orphans.