Warning: The following contains SPOILERS for Legends of Tomorrow season 6, episode 8, "Stressed Western."
The Legends of Tomorrow episode "Stressed Western" featured a mysterious cowboy, who described the events of the episode in song - but who is this mysterious figure? While the character originally seemed to be a narrative device meant to fit the episode's Western theme, "Stressed Western" ended with the cowboy being kicked off of the Waverider after it became apparent the Legends were aware of his presence. This raises the question of just who this cowboy was, and how he knew so much about the Legends despite apparently being an ordinary denizen of Fist City in the year 1891. One explanation suggests itself immediately within the context of the larger Arrowverse: the cowboy narrator is from the 5th Dimension.
In the world of DC Comics, the inhabitants of the 5th Dimension (commonly called Imps) have the near-magical power to alter reality in the physical world. Thankfully, most of them have little interest in the mundane third dimensional world most superheroes inhabit. Despite this, a few of them (most notably Bat-Mite and Mr. Mxyzptlk) occasionally manifest in the real world and cause trouble, either through honest efforts to help the mortals they've fixated on, or to amuse themselves with mischievous pranks.
A number of 5th Dimensional beings have appeared in the Arrowverse, with the most recent being the troublesome Princess Nyxlygsptlnz (aka, Nyxly) in Supergirl season 6. Thankfully, most of the Imps who have manifested in the Arrowverse have been less murderous than her. Some of them were outright benevolent and tried to use their phenomenal cosmic powers to aid heroes whom they respected. This seems to fit the behavior of the singing, fourth-wall-breaking cowboy from Legends of Tomorrow, who seemed to ire the time-traveling heroes, even after Ava Sharpe declared that "we can't keep the cowboy narrator!" and forced Nate Heywood and Behrad Tarazi to man-handle him off of the Waverider.
Mr. Mxyzptlk in Supergirl
The most famous of the many Imps to torment the heroes of Earth, Mr. Mxyzptlk was a frequent foe of both Superman and Supergirl in the comics. The first Arrowverse incarnation of Mr. Mxyzptlk (played by Peter Gadiot) appeared at the end of the Supergirl season 2 episode "Luthors," before going on to be the chief villain of the next episode, "Mr. & Mrs. Mxyzptlk." It was here that "Mixy" used his god-like powers to try and force Supergirl to marry him, in a largely ludicrous adventure based on an old Supergirl comic. Thankfully, Supergirl was able to trick Mr. Mxyzptlk into spelling his name backwards; a unique trigger that forced him to return to the 5th Dimension, for reasons that only made sense under the strange laws governing Imps.
Mr. Mxyzptlk returned in the Supergirl season 5 episode "It's A Super Life," which was also the 100th episode of Supergirl. Played by Thomas Lennon, Mixy claimed to have seen the error of his ways and been ordered by the Court of the 5th Dimension to help everyone he had ever hurt with his magic. This led him to take Kara Zor-El on a walk down memory lane, offering to help her find a way to change her past so that her damaged friendship with Lena Luthor might be salvaged. In the end, Kara decided that there was no way to change her past without risking making the future worse and that she should own her mistakes and work to improve herself, as Mixy had. Kara and Mixy parted as friends, with Mixy thanking Kara for giving him the chance to prove that he had changed for the better.
Even in his kinder, gentler form, Mr. Mxyzptlk's behavior was typical of most 5th Dimensional beings. Easily bored and prone to jokes, Mixy could be said to be basically good-hearted, if somewhat selfish, like an overgrown child. Even at his worst, his major motivating force was a need to be loved, leading him to act out for the approval of the people watching his antics. He also professed an honest iration for The CW's Supergirl, though the showman in him couldn't help but try to express that iration by teasing her as he tried to impress her.
The Music Meister and the Musical Episode of The Flash
Supergirl encountered another being with similar reality-altering powers at the end of the Supergirl season 2 episode "Star-Crossed." This led into a musical crossover event, which unfolded in The Flash season 3 episode "Duet," where Barry Allen was abducted, along with Supergirl, and forced to perform in a psychic landscape created by a cosmic being who called himself the Music Meister. Played by Darren Criss, the Music Meister was a big fan of musical theater and delighted in forcing Kara, Barry and several other familiar faces plucked from around the Arrowverse to act out a drama of his own design.
While the Music Meister had threatened Kara and Barry's lives and the lives of their significant others, Iris West and Mon-El, he eventually revealed that there was method to his madness. He explained that he was a big fan of heroes and that he had noticed that Kara and Barry were heartsick and questioning their love. This led him to set up his little show to teach them both that while they may save people as superheroes, they needed to let themselves be open to being saved in matters of love by other people. While it was never confirmed just what exactly the Music Meister was, many Arrowverse fans have commented that his powers and motivations resembled those of a benevolent 5th Dimensional Imp.
Theory: The Cowboy Narrator Is An Imp
With a love of singing and performing equal to that of the Music Meister and an iration for the Legends reminiscent of Mr. Mxyzptlk's crush on Supergirl, it seems likely that the cowboy narrator might be a 5th Dimensional Imp. Certainly it would explain why, throughout the Legends of Tomorrow episode "Stressed Western", the singing troubadour seemed to know everything there was to know about the Legends, despite their not being forthcoming about their identities when they arrived in the town of Fist City. It would also explain his apparent awareness of the audience watching him and his ability to teleport around the Waverider at will while summing up the ending of the episode, without skipping a beat.