Time remnant

"It's possible Eobard was in the Speed Force, protecting him like a bomb shelter, keeping him alive and his timeline intact. It's what's known as a “timeline remnant”."

- Harry Wells on Eobard Thawne's time remnant

Time remnants or timeline remnants are a phenomenon that occurs as the result of meta-human speedsters' time travel abilities. Due to possible time raptures and changes brought by the speedsters traveling (and staying) further back in time (for example, in case of a speedster's death) the time remnants, preserved by the Speed Force as if in their original timelines, have to appear in the new timeline, to prevent further time paradoxes.

Overview
The time remnants seem to appear when an aborted timeline's events have to happen for the current timeline to exist, but would technically never happen, because a speedster's actions led to them being erased from existence. This happened to Eobard Thawne, who originally fought Barry Allen in the 2020s as the Reverse-Flash, then restarted the timeline in the year 2000, so that the particle accelerator explosion, as well as Barry Allen's ascent to the mantle of the Flash, would happen in 2014; only for Eobard to be erased from existence due to the suicide of Eddie Thawne, his ancestor. However, since Eobard Thawne's existence and the changes to the timeline that he brought (the death of Harrison Wells and Eobard's subsequent masquerade, the explosion of the S.T.A.R. Labs particle accelerator, the creation of the Flash in 2014) had to happen for Barry to fight him in 2015 and for Eddie to decide to sacrifice himself in the first place, the Speed Force has preserved a younger version of Eobard Thawne in form of a time remnant, who had to appear for a "first encounter" with the Flash in 2016 to preserve the events of Eobard's personal timeline (even though the Thawnes past Eddie were no longer supposed to exist, and the original Eobard himself would only be able to encounter the Flash after 2020). The explanation as to why the "Evil Wells's" time remnant was preserved in the new timeline came from Harrison Wells from an alternate Earth.

The mechanics regarding the appearances of time remnants of other speedsters, such as those of Hunter Zolomon and Barry Allen, are not explained in such a complex manner as Thawne's, but it can be presumed that their origins are similar: they are the results of an aborted timeline. All that is known is that after Barry Allen decided to go to Earth Two, Hunter Zolomon traveled into the past, arranged for a meeting with a time remnant of his, and convinced him to die on Earth One before the eyes of Team Flash, ensuring that Barry would want to increase his speed and avenge the man he knew as "Jay Garrick".

Time remnants, unlike, for example, the "clones" of Multiplex, seemed to act completely separate from the "main" or "current" version of the person, and required convincing to allow themselves to be sacrificed.

It is unknown what would happen to a time remnant if they realized that the timeline they come from no longer exists, or that they can't exist any longer themselves either. Presumably, they would dissipate, like what happened to a 2015 version of Barry Allen, after he saw the 2016 version of Barry Allen save Nora Allen and defeat Eobard Thawne, changing the timeline.

History
Presumably, the first time remnant that Barry Allen witnessed in his own current timeline was the Flash with the white symbol on his chest that Eobard Thawne fought at Allen House in 2000s. Since this Flash was supposed to be Eobard's original enemy, he had to originate from a timeline in which the Flash was created in 2020 and had his both parents stay alive, which was aborted after Eobard Thawne killed Barry's mother and was left stranded in the 2000s, disguising himself as Harrison Wells. The white-symbol Flash stopped the 2015 version from trying to fight Thawne and save his mother.

In 2016, a time-traveling Eobard Thawne appeared in Central City, even though he was supposed to be erased from existence. Harry Wells explained his continued existence as that of a "timeline remnant", whose personal timeline events had to happen to ensure the existence of the current timeline. The Flash captured Thawne but, realizing that leaving him imprisoned forever in 2016 would lead to negative effects on the current timeline (shown to be true when Cisco Ramon, Vibe, started bleeding and later dissipating into an incorporeal form), had to help him resume his time travel, as Thawne's time remnant entered a wormhole into a supposedly erased timeline that he hailed from. It was implied that Thawne's time remnants would appear in the future, to fight the Flash and, at one point, to travel back in time to the year 2000, to kill Barry's mother, "become" Dr. Wells and ensure the time loop.

Hunter Zolomon, a speedster from Earth Two who masqueraded as both the hero, Flash, and the villain, Zoom, to give people hope and then take it away from them, also found out about the phenomenon of time remnants on his own, and sought do abuse it, giving him an advantage in his schemes. Introducing himself to Team Flash as "Jay Garrick", he became a friend and mentor to Barry Allen, and a love interest of Caitlin Snow. However, after Barry returned from his adventures on Earth Two, "Jay" seemingly died at the hands of "Zoom", before the eyes of the team.

After his identity as Zoom was revealed, Hunter Zolomon was questioned by Iris West and Joe West on the reasons for his charade and the fact that they saw him die. Hunter explained that, after Barry Allen has closed all but one of the breaches to Earth Two and decided to travel there and save Jesse Quick from Zoom's lair, Hunter realized that he'd need to potentially be on both Earths at once during the ensuing crisis. With this realization, he traveled back in time and, after an unknown sequence of time travel, convinced a time remnant of his to come to Earth One and sacrifice himself, giving Barry Allen extra motivation to avenge "Jay" and stop Zoom, increasing his speed in process, only for Zoom to siphon the Speed Force from him. Convincing his time remnant to die wasn't easy, but once he saw the genius in his current self's plan, he agreed to it.

Hunter Zolomon killed another time remnant of himself when he fought Barry Allen, taunting him about his father's death, and showing Barry just how powerful a speedster can be if they abandon fear and mercy and stop at nothing to achieve their goals. His lesson learned, Barry Allen later used the help of his own time remnant during his "race" with Zoom at the Magnetar. Said time remnant agreed to sacrifice himself to stop the Magnetar's effect on the multiverse. Averting Zoom's plan to destroy all alternate Earths' and their timelines sans that of Earth One, Barry Allen defeated Hunter Zolomon and left him for the Time Wraiths to capture, for the danger Zoom posed to the timeline.

Presumably, after Barry decided to time travel back to the year 2000 to stop Eobard Thawne and save Nora Allen, he met two versions of himself that would become time remnants: the original timeline version of Barry Allen from 2020s, who fought the Reverse-Flash, and the year 2015 version, who traveled back in time after agreeing to the deal from Eobard Thawne, but made sure not to change the timeline's events. After seeing his mother live, the 2015 Flash dissipated into thin air.

Powers and abilities

 * Time Travel: the existence of time remnants is dependent on the phenomenon of time travel, and they have to possess the ability to travel through time (or be helped by someone with such an ability) to continue serving in their roles. For example, Eobard Thawne's time remnant had to be assisted by Barry Allen to ensure that he continues his time traveling and eventually fulfills his role in the creation of the current timeline.
 * Speed Force Conduit: all time remnants seen so far come from time-traveling versions of different speedster meta-humans. According to Harry Wells, the Speed Force serves as a "bunker" that shields the speedster from deaths of their past selves.

Trivia

 * Although the phenomenon was originally named "timeline remnant" in The Reverse-Flash Returns, the fan nickname of "time remnant" was later made official in the episode Versus Zoom.

Behind the scenes

 * In DC Comics time remnants indeed appear in speedster and time travel stories. However, the closest example to the one showed in the show would be the appearance of a younger version of Eobard Thawne during the w:c:dc:The Flash: The Return of Barry Allen comic arc. After undergoing surgery that has given him the appearance identical to that of Barry Allen, Thawne traveled to the past to meet and befriend his idol, only to appear sooner that he intended to be, and to see the Flash Museum's exposition on him detailing his appearance as the villainous Reverse-Flash and subsequent death at the hands of his idol, Barry Allen. Realizing that his idol was the one to kill him, Thawne experienced PTSD and convinced himself that he is not actually Eobard Thawne, but rather Barry Allen reborn. Even after Wally West, the Flash, showed him the truth, Thawne wanted to stay in West's era, to ensure that he never travels through time and thus never allows Barry Allen to kill him in the first place. To keep the timeline intact and preserve Barry's memory from Thawne's continuous villainous actions in his name, West had to trick the Reverse-Flash into reclaiming his identity and being sent back to the future, ensuring that Thawne would still go back to Barry's era and fight him as the Reverse-Flash, like he was supposed to.
 * It can be argued that in Flash Back we see Barry Allen himself become a time remnant, whose time-traveling had to happen to ensure that the current timeline, with Hartley Rathaway an ally of Team Flash and the creator of a weapon against the Time Wraiths, would exist, and the old timeline, with Hartley as an unrepentant villain, would be erased. Cisco Ramon's knowledge of the Flash's white chest symbol also came differently in the the pre-altered version of the timeline, though, ironically, also due to Barry's meeting with a time remnant from 2024.