Genji Shimada is the son of Sojiro Shimada and the brother of Hanzo Shimada. His father was the leader of the Shimada clan which is a very popular Yakuza situated in Japan. After Sojiro Shimada would retire or in the worst case pass away, one of his sons would replace him on the helm as the leader of the Shimada clan.
Genji even his younger days was a very skilled swordsman but was constantly outshined by his older brother, Hanzo who was preferred more by the elders of the Shimada clan. After many years of trying to impress his father and the clan, Genji was constantly outperformed by his brother who was set to replace his father as the leader of the Shimada clan when the time came.
Eventually, Genji lost interest in the clan and started enjoying his life more. Because of his ignorance and show of disgrace towards the Shimada clan, the elders of the clan decided to have Hanzo kill his brother, who did exactly that. After Genji was supposedly killed by a reluctant Hanzo, his body was found by Overwatch who brought him back from near death.
Overwatch’s engineers and doctors worked together to heal Genji and replace his damaged body parts and organs were replaced with cybernetics which made him the perfect soldier who was tireless, never needed much rest and felt no sympathy for his enemies.
Overwatch’s lead commander Jack Morrison did not know what to do with him but Blackwatch head, Gabriel Reyes is now known as Reaper, opted to recruit him for his team where he joined Jesse McCree and Moira along with Gabriel Reyes himself.
During his time with Blackwatch, Genji went on many missions and adventures with Blackwatch completely secret from the world, where he dealt with many corrupt leaders, illegal weapon suppliers, and Talon’s allies in ways Overwatch could not make public. Genji did his missions, mostly in complete isolation and silence from his group, not saying a single word or letting anyone help him.
However, it is shown that Genji had developed a good friendship with the cowboy of the group, Jesse McCree, who he shares many stories and moments with during his time in Overwatch. In interactions with the two characters, Genji made clear that he always had his suspicions regarding Reyes and Moira which turned out to be right.
In an Overwatch comic, he said that he felt evil energy emerging from the inside of Moira and always showed fear when around her, thinking that her actions were unpredictable and irrational. Genji always looked up to Reyes because Reyes never took no for an answer and got work done no matter what. Much like everyone else Genji was quite surprised with Reyes’ defection despite knowing his rogue nature.
Genji had a good run with Blackwatch which didn’t last so long. He had gained respect throughout the entire Overwatch regiment because of consistent success in all of his missions. His final mission, however, did not go as great. Blackwatch was deployed in Venice, Italy, 8 years before the events of the full game, they were sent to capture and detain Antonio, a quite famous arms dealer who illegally dealt weapons and ammunition to Talon as well.
When the team encountered Antonio, he mocked them saying that if you take me the world will know about Overwatch’s covert team and his lawyers would have him out in a week. Reyes took a minute to think before saying you’re right and proceeded to shoot Antonio, causing him to die and his corpse to fall off the building.
The whole found out about Blackwatch because of this causing Blackwatch to disband, this was the first step towards Overwatch being disbanded. Genji continued to serve work with Overwatch as shown in the Storm Rising event, it is unclear what happened in-between then and Overwatch being disbanded.