Reverse-Flash

"I saw what looked like a ball of lightning. Inside the lightning there was a man."

- Barry Allen

An unnamed man, often referred to by such names as the "man in yellow", is the murderer of Nora Allen, and an apparent metahuman.

Biography
The man (a yellow blur, accompanied by an unidentified red streak) circled Nora Allen, before stabbing her directly in the heart with a knife. He took Barry blocks away and it seemed to police as if Henry Allen had murdered her.

Some fourteen years following, the man threatened the life of Iris West by stabbing a similar knife into her photograph, to scare her father away from investigating the Nora Allen murder. This happened after Joe apologized to Harrison Wells for suspecting him earlier of involvement.

Trivia

 * The man in yellow resembles the suit worn by various characters called the "Reverse Flash" (first "Professor Zoom", then "Zoom"), villains who share the Flashes' super-speed.