Kara landed on Earth 17's Metropolice from her planet Daxam and was adopted with name Kara Gray.
Barry is born speedster. His maternal grandfather, Jay Garrick was known as The Flash.
At some point they became superheroes and joined the Legion of Super-Heroes. They became best friends.
During the fight with Hellgammite Mr Mxyzptlk accidentally opened a portal which transferred Kara and Barry to Earth 1.