Sōichi was a former Border Agent, and one of Border's founders. He also was Yūgo's rival. He used to be Jin's mentor. He used his life to make a Black Trigger, the same trigger which Jin formerly used.
Having taught Jin, one of Border's most powerful agents, how to fight with Triggers, and having been Yūgo's rival when the latter was as strong as Shinoda currently is, Mogami was undoubtedly an extremely skilled fighter. His ability to create a Black Trigger implies that he possessed a very large amount of trion.
- In a photograph of Old Border, a fair-haired man is depicted who many fans speculate could be Mogami, due to his stading next to Shinoda and Jin. His sunglasses and slicked back hair are also similar to what Jin will wear later on. This would be consistent with Yuri's explanation to Osamu, as she revealed that the man in the photo died a little over five years prior to the current events.