The Flash is the fastest man alive. The protector of Central City and Keystone City, fighting against evil using his super-speed and a dedicated sense of heroism. His legacy, the Flash Family, spans throughout history tapping into the enigmatic Speed Force to gain their powers. Jay Garrick was the original Flash of the Golden Age, and a founding member of the Justice Society of America. Barry Allen was the Flash of the Silver Age and a founding member of the Justice League of America. He sacrificed himself to save the universe during the Crisis on Infinite Earths, but was resurrected during Final Crisis. His replacement was Wally West, his nephew and the original Kid Flash, formerly of the Teen Titans. The fourth was Bart Allen, who started his career as Impulse.
The Flash was created by Gardner Fox and Harry Lampert in his original incarnation. He first appeared in Flash Comics #1 (1940). The modern version was created by Robert Kanigher, John Broome and Carmine Infantino, first appearing in Showcase #4 (1956).
During E3 2015, many spherical models symbolizing various worlds displayed a variety of characters interacting with one another. The Flash appeared in the Back to the Future sphere, though it is unknown why he appeared, due to his lack of a physical character appearance in a LEGO Dimensionsset.
Because of this, it's possible that he may have originally been planned as a playable character but was scrapped for unknown reasons.
It is also possible that he could still be released in the future at some point, possibly being included in the 71341 Polybag for Year 2.