One of the most popular characters on Arrow is John Diggle, the former soldier that was hired by Oliver's mother to monitor him after he returned back to Star City after being on Lian Yu. Unlike most of the Arrowverse characters that were taken directly from the comics, John Diggle was an original character created specifically for the show.
As the show continued, Diggle evolved as a character, forming a strong bond with Oliver and eventually becoming a hero himself. By the end of the series, he was a much different character than the one he was when he was first introduced.
Learned Oliver's Secret
When Diggle was first hired as a bodyguard, he struggled to look after Oliver who would constantly use every opportunity to ditch him so that he could operate as the Hood.
In episode 4 of season 1, "An Innocent Man", Diggle learns about Oliver's secret life as the vigilante after he gets shot by Deadshot and Oliver brings him back to his hideout, saving him while still in costume. Oliver would then go on to explain his mission to Diggle who, after thinking it over, agrees to help.
Became A Vigilante
After learning that Oliver was really "The Hood", Diggle began to help Oliver out in the field. Initially, he would serve as a decoy or help collect information that they could use in taking down Star City's criminal elite.
Additionally, Diggle's military and martial arts training made him a great asset with being the first official member, besides Oliver, of what would later be known as "Team Arrow".
He Got Married
Diggle is currently married to Lyla Michaels, a member of and later director of the organization ARGUS. The pair first met each other when they were both serving with the US Army in Afghanistan which is where they got married for the first time.
They would later get a divorce due to their relationship being difficult to maintain while they were civilians. They later reconnected when Diggle sought the assistance of ARGUS in finding Floyd Lawton aka Deadshot. They later got re-married in season 3 not too long after the birth of their daughter, Sara.
He Had Kids
Together, John and Lyla had a child together named Sara, who was initially erased from the timeline by Barry due to the events of Flashpoint, and replaced by a son, John Diggle Jr. However, later on account of Oliver's sacrifice in Crisis on Infinite Earths, Sara was brought back, allowing the two to co-exist.
Viewers also learned, thanks to the future flashbacks in season 7, that Diggle would eventually go on to adopt Connor Hawke, the son of Ben Turner aka Bronze Tiger in a potential future timeline.
He Got A Suit And A Codename
After going out with Oliver in the field on numerous occasions, he was finally given the official codename of "Spartan", a direct reference to the drills his step-father used to make him and his brother do, as well as his own uniform and helmet in the show's 4th season.
While the original look would be updated various times over the years to add more colors and alter the material, the general design of the outfit remained the same.
He Went To Prison
After reenlisting and while on a mission with his men to retrieve a piece of a bomb from Genesis Day, Diggle found himself in a shootout that he learned was designed to falsely accuse him of trying to steal the weapon.
He was then sent to a military prison where he started to have hallucinations of Floyd Lawton aka Deadshot. He insisted to Lyla that he remain there, clearly still blaming himself for killing his brother, before ultimately being rescued by Oliver and returning to Star City.
He Got Injured
At the end of season 5, Prometheus captured most of team Arrow and held them on Lian Yu before blowing up the island. While William's mother, Samantha, died, the rest of the main characters survived and were able to make it back to Star City.
While they were able to make it out alive, that didn't mean it didn't have a lasting effect. For one, as a result of being hit with bomb shrapnel, Diggle suffered severe nerve damage preventing him from being able to fully control his arm and keep it steady. Since it was keeping him from being at his best in the field, he turned to drugs to help him and was also given an implant created by Curtis to help him recover.
He Temporarily Became Green Arrow
After a fake photo was released that seemingly exposed Oliver as the Green Arrow, he asked Diggle to take on the role in his place. Unlike Oliver, who usually used a traditional bow and arrow, Diggle instead a crossbow which was much more similar to his usual weapon of a handgun.
He would eventually be forced to give up the mantle when Oliver learned about his injuries and reprimanded him for putting the team in danger.
He Ended Team Arrow
Another thing that happened as a result of heroic sacrifice at the end of the crossover was that reality was rewritten in several major ways. The biggest way is that there was no longer any crime in Star City.
Due to that, the city no longer had a need for vigilantes at all, so Diggle and the rest of the team made the choice to disband Team Arrow seeing as there wasn't much they go do for the city wearing masks. He along with the rest of the members decided to go their own separate ways, with him and his family deciding to move to Metropolis.
Got Green Lantern Ring
For years, there was a theory going around that John Diggle was actually the Arrowverse version of John Stewart. That theory was seemingly confirmed in the series finale of Arrow when John was packing up his family and preparing to go to Metropolis when a mysterious obvious fall from the sky and creates a small crater.
When Diggle goes to see what it was, he discovers a small box with a glowing green object inside. Fans never actually saw what was inside the box but the answer seems pretty obvious.