Obviously, The Flash and Supergirl themselves are very popular. Their unique powers and personalities make them central to most fanfictions.
Another popular character could be Reverse - Flash. His antagonistic relationship with The Flash creates a lot of drama and tension in fanfictions. He can be used to create complex and exciting story arcs that often involve time - related mysteries and battles.
Well, obviously Supergirl and The Flash themselves are very popular. They are the main characters in their respective shows and fans love to write about them in different scenarios. Also, villains like Reverse - Flash are often popular in fanfiction as they add a lot of conflict.
One popular storyline is when Flash and Supergirl team up to fight a common enemy from another dimension. This allows for a lot of action and showcases their different powers working together.
Well, obviously Superman and Supergirl are the most central characters. But other popular characters often include Lex Luthor. He is always a great antagonist, constantly devising new plans to take down the Kryptonians. His intelligence and deviousness add a lot of tension to the stories.
Well, Cisco Ramon from The Flash would be a great addition. He could use his tech skills to create some new gadgets or figure out ways to counter the villains' powers. Also, Martian Manhunter from Supergirl's side. His shape - shifting and telepathic abilities would add a whole new dimension to the story.
One popular storyline is where Flash and Supergirl team up to fight a common, powerful enemy like a super - villain from another dimension. They combine their speed and strength in unique ways to save the world. Another is a more character - driven plot where they develop a romantic relationship. Flash's quick - witted charm and Supergirl's strong - willed nature make for an interesting dynamic. There are also storylines where they swap powers temporarily, leading to humorous and action - packed situations as they learn to use each other's abilities.
Kara Danvers (Supergirl) is also very popular. Her superpowers and kind - hearted nature make her a central figure. Fans love to see her interact with the more grounded characters from Arrow, like when she tries to understand their more human problems despite her god - like powers.
Barry Allen, the Flash himself, is obviously very popular. His speed and his kind - hearted nature make him a central and beloved figure.
Reverse - Flash could be really interesting. He could manipulate time in ways that not only affect Flash but also Supergirl. For example, he could go back in time and prevent Supergirl from arriving on Earth, which would change the whole timeline. Flash would then have to convince her to come to Earth again while also dealing with Reverse - Flash.
Lex Luthor could be a great villain. He might be jealous of Supergirl's powers and see Flash as an obstacle in his plan to defeat her. He could use his scientific knowledge to create some sort of weapon that targets both speedsters and Kryptonians.
The Flash himself, of course. He is the central character and his ability to time travel is what drives most of the stories. Fans love to explore his different adventures through time.