Jimmy Robertson has played a crucial role in Bosch: Legacy, but truth be told, he has been an important part of the story for much longer. He was first introduced in Season 3 of the original Bosch series and has appeared on the show a few times since then.
Jimmy is seen again in Bosch: Legacy as he investigates claims by Preston Borders, who alleges that he killed Kurt Dockweiler on Harry Bosch's orders. However, this is not the first time Jimmy has believed Harry to be guilty of murder. Back in Season 3 of Bosch, Jimmy was convinced that Harry had killed Edward Gunn, though later, he was proven wrong. Therefore, it can be established that Jimmy has been connected to the series way before Bosch: Legacy.
Jimmy Robertson in Bosch Before Bosch: Legacy
Before Bosch: Legacy, Jimmy Robertson had already been a recurring character in the original series. In the latter's third season, Harry Bosch had investigated a case in which Edward Gunn was a primary suspect. However, at that time, Harry could not prove Gunn to be the culprit and put him in jail. Nevertheless, he had decided to keep his tabs on Gunn and had flagged his file.
When Edward Gunn was found dead, Jimmy Robertson was assigned the case. Harry arrived at the scene, but it was not his case, so he had his hands tied.
However, the situation became complicated when Harry's prints were found at Gunn's apartment. Jimmy was convinced that Harry had killed him out of his frustration for not being able to put him behind bars. On the other hand, J. Edgar conducted a separate investigation and proved that Harry was being framed. However, even though Harry was not the killer, he had been present when the killers broke in to kill Gunn.
Though Jimmy Robertson was not pleased with Harry's involvement in the Gunn case, he accepted that Harry was not the murderer. The two continued to maintain a strained but professional relationship in the season that followed.
Jimmy later returned to Bosch to investigate Howard Elias' murder, another case that made him work in close collaboration with Harry. Jimmy returned to the show one last time in season 7 to investigate the attempted murder of Honey Chandler.

We finally see Jimmy Robertson return to the franchise in the third season of Bosch: Legacy when he investigates Preston Borders' claim. At the end of Bosch: Legacy Season 2, Maddie receives a call from Borders, wherein he tells her that he had killed Kurt Dockweiler on Harry's orders.
The accusation prompts an investigation, and Jimmy steps in to do the same. Over the course of the season, Harry is proven innocent, and it becomes evident that Preston had tried to frame him. It is revealed that his real plan was to accuse Harry of murder so as to trigger a court hearing. This would have helped Borders find an excuse to come outside of the prison so that he could attempt an escape. But Harry was already suspicious and prepared, and thus prevented Borders' escape. The attempt ends in a deadly shootout, killing Borders, and closing a very dark chapter in Harry's life.
The first six episodes of the third and last season of Bosh: Legacy are currently streaming on Amazon Prime Video. The last four episodes will be released as two double-episode releases on 10th and 17th April 2025.
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