Actor: Grant Gustin
In Season 1 it is revealed that Barry often ran from bullies like Tony Woodward, throughout his childhood- but there is no clear explanation as to why. Based on what he told his mother, in the episode "Pilot", it's safe to infer that he felt compassion for other children that were being picked on and made the resolve to stand up for them.
“Those guys were picking on kids, just ‘cause they thought they weren’t cool. It wasn’t right...I guess I wasn’t fast enough.”
-Young Barry Allen to Nora Allen
Nora Allen responds to her clearly distraught son that "it's better to have a good heart than fast legs." and when his father enters the room Nora tells him that Barry got into a fight and won, to which Henry Allen responds "way to go slugger...oh and uh no more fighting."
However, that night after Barry has gone to bed he's awoken by a strange noise coming from downstairs. He ventures down to the living room where his mom is yelling for his dad and kneeling on the floor, surrounded by two rapidly moving red and yellow colored streaks of light. He attempts to move closer, shouting for his mother the whole time, but she warns him not to "let him touch you." Just as Henry enters the scene and tells Barry to run, he's whisked away outside. On Season 2 Episode 8 of Arrow, "The Scientist", Barry mentions that in an instant he was twenty blocks away from his house. When he finally gets back, his mother is dead and his father is being taken away for her murder.
After that, Detective Joe West and his daughter Iris West take him in because they were close with the Allen's. No one believes Barry's stories that his dad didn't murder his mom but a person inside a blur did, so for a long time he felt lost and abandoned. They all thought he was trying to cover for his father. Several times as a child, he ran away from the West's home to visit his dad in prison.
For the remainder of his childhood he stayed at the West house. His dad at one point wanted him to change his last name so people wouldn't know they were related- but Barry responded that he was proud of his last name and that he was glad that people knew. Joe and Iris taught him to fight (more or less) and Barry was integrated into their family. Somewhere after he graduated college he moved out into the little room above the Central City Police Department where he did most of his CSI work, but after Iris moved out to live with Eddie he returned back to the house to give Joe some company.
Becoming the Flash
Barry Allen first was shown becoming the Flash on Season 2 Episode 9 of Arrow, "Three Ghosts", at the very end of the episode. The night he returns from Starling City he takes Iris to go see the Particle Accelerator, built by his idol, Dr. Harrison Wells , turned on for the first time. However, once there a teenage kid manages to steal Iris' bag which contains her laptop resulting in Barry pursuing the thief. The kid manages to hit Barry in the stomach after hiding behind a building corner, but is ultimately apprehended by Eddie Thawne, a police man who recently joined Central City's ranks. Barry retires back to his lab, though it isn't for sure why, and proceeds to watch the Particle Accelerator go online from his computer. The power shuts off as the storm outside increases, and Barry looks up just in time to see some sort of explosion and bolts of what appear to be lightning burst out of S.T.A.R. Labs. He then goes over to his ceiling window and begins to open it but gets distracted by the elements and liquids he keeps, floating from their bottles and into the air. A bolt of lightning then crashes through the glass of the window and strikes Barry, flinging him backwards into one of the shelves and rendering him unconscious. A ripple of electricity sparks along the inside of his cheek, a first clue of what he's to become once he wakes.
After nine months in a coma, Barry wakes to Cisco Ramon and Dr. Caitlin Snow playing Lady Gaga's Poker Face, which Cisco insisted Barry liked from stalking his Facebook page. Caitlin immediatley asks him to urinate in a plastic container to which Cisco responds "not this second" and goes on to tell Barry that he was struck by lighntning and moved to S.T.A.R Labs. Harrison Wells takes Barry through the facility and explains that the night of the Particle Accelerator many people were severly injured, including Wells. He explains that the electron volts became unmeasureable and energy from the detonation was thrown into the sky, which in turn seeded a storm cloud. Barry makes a hasty exit, thanking them for saving his life (and asking to keep the S.T.A.R. Labs sweatshirt he was given to wear) and telling them that he feels normal now.
Barry immediatley heads to CC Jitters, a local coffee shop, where his love interest and best friend, Iris West works. While talking to her about how his heart kept stopping back in the hospital, a waitress behind Iris drops her tray. Barry witnesses it fall in slow motion. He covers his shock fairly well before going over to the Central City Police Department. There, he stops a criminal from reaching for a police man's gun- but so quickly that no one saw him do it. Confused, Barry goes outside to a back alley to test his newfound abilities.
- The reporters name when the Particle Accelerator malfunctions is "Linda Park." But we see Linda Park again later in the season as an obviously different person.
- Barry Allen was first introduced to the CW's DC universe on Season 2 of Arrow.