The Flash has become a pillar of the ever-expanding Arrowverse on the CW Network since its debut in 2014. Over the course of its first six seasons, the show has introduced hundreds of characters from the greater DC Universe, but a few of them are actually inventions for the show, not unlike Harley Quinn.
Some characters have migrated into the comic books, such as Gregory Wolfe, the corrupt warden of Iron Heights Prison introduced in season one. He debuted in the comics in 2017, but some still remain unique to the show. Here is every major character from The Flash who isn't in the comic books.
10 Joe West
Joe West, the father of classic comic book character and Flash love interest Iris West, is actually an invention for the series. A CCPD detective and legal guardian to Barry Allen after the death of his mother at the hands of Barry's father, Joe West is a great addition to the Flash mythos. He provides more depth and background to Iris and creates some interesting dramatic tissue between Barry and Iris as their relationship evolves. Naturally, he has many other alternate versions in other realities, just not in the comics.
9 Eddie Thawne
Iris' boyfriend as the series opens is actually Joe West's partner in the Central City Police Department, and another wholesale invention for the show. Eddie Thawne heads up the task force to find the public menace known as the Flash and while he doesn't know Barry Allen is the Scarlet Speedster, he doesn't like him, either. The two clash over their interest in Iris, but ultimately Barry wins out. Tragically, Eddie dies by his own hand, trying to stop his evil descendant Eobard Thawne, The Reverse Flash, from ever coming into existence.
8 Harrison Wells
Eobard Thawne initially disguises his identity in the present as Harrison Wells, a character who doesn't exist in the comic books. Wells is the power and intellect behind S.T.A.R. Labs, the central hub of the series (though it doesn't have a lot to do with The Flash in the comic books). The evil future Thawne kills the real Harrison Wells and then assumes his identity, something that really isn't explained until flashbacks that fill in some of the blanks as to what happened in the crazy first season of the show.
7 Eva McCulloch
Eva McCulloch doesn't exist in the comics, either. Well, not quite. Eva is unique to the show, but she is based on a character from the comics, Evan McCulloch. This quantum engineer and co-founder of McCulloch Technologies is pretty different from the comic Mirror Master, making her essentially a brand new character.
Though Eva does turn out to be a Mirror Master in the truncated sixth season of The Flash, she differs in dramatic ways. The comic version is a man and a Scottish mercenary whose tragic backstory sets him on a path of violence that doesn't have much to do with pocket universes until much later.
6 Mason Bridge
Mason Bridge is another character from the first season of the series created to help build out the world of Iris West. Bridge mentored Iris at Central City Picture News who, like any good reporter, smells a scoop with Eobard Thawne. He starts digging around and finds out that Harrison Wells isn't who he says he is, but he is a total murderer. Bridge pays for this information with his life, but his death ultimately leads Iris, her father, and Barry toward the truth and a confrontation with Reverse-Flash.
5 Tracy Brand
Tracy Brand joined the show in the third season. A doctoral student who studies speedsters for her thesis, she becomes the target of the villain Savitar since she helps defeat him in the future. Savitar originated in the comics, but was changed pretty drastically for his television adaptation. Tracy remains a live-action feature of The Flash mythos. Later on, she helped bring Barry Allen back from inside the Speed Force.
4 Kamilla Hwang
Kamilla Hwang (portrayed by Victoria Park) arrived in Central City in season five. She's initially a bartender but then gets pretty involved in the world of the show by taking on jobs at both S.T.A.R. Labs and the Central City Citizen alongside Iris West-Allen. She plays a bigger role in season six, as Iris' investigation of what turns out to be the Mirror Universe creates some drama and intrigue in the office (as in Kamilla gets trapped in it). Unfortunately, it got cut short due to the pandemic shutting production down, but hopefully Kamilla returns next season.
3 Gideon
Gideon is a unique character the show created who would certainly make a good addition to the comic books. Barry Allen created this A.I. assistant in the original future timeline. But then Eobard being Eobard, he co-opted the program for his own ambitions, which involved basically murdering everybody.
Morena Baccarin voices the character, and it's not her first rodeo in superhero fare. She also starred in Gotham and of course Deadpool. The character has another version integrated into the system of the Waverider over on Legends of Tomorrow.
2 Black Siren
Black Siren doesn't exist in the comic books, but she is – in a way – an offshoot of a comic character. Black Canary is a major figure in DC Comics and Black Siren is just one of the many versions that exist within the Arrowverse. In The Flash, Black Siren is the Earth-2 version of the character who is one of the villain Zoom's lieutenants. This character doesn't exist in the comics, but a character named Black Siren does feature in the "Legacy" episode of the Justice League animated series, a fictional hero who provides one of the show's most poignant moments.
1 Nora West-Allen
One of the best original characters for the television series is Nora West-Allen, the grownup daughter of Barry and Iris from the future. While her chosen superhero codename XS has its origins in the comics (with another character – Jenni Ognats, granddaughter of Barry Allen, and a member of the Legion of Super-Heroes) Nora is new to the show. Nora is ultimately erased from existence, though her eventual birth seems likely. That would be a different version of her though, but then again, everyone is kind of a different version of themselves in the show.