Barry, like the player, lives in Twinleaf Town and is also the player's childhood friend (and soon-to-be rival). Barry is always in a hurry and extremely impatient, and because of this, often crashes into other people (usually demanding a fine from them) or misses important details.
Barry is incredibly determined and it is very difficult to discourage him; the only thing that has done this so far was his crushing loss to the Team GalacticCommander, Jupiter at Lake Acuity, although this defeat lead to a major positive change in Barry's character, causing him to become slightly more serious, realistic, and mature. He is also quite the braggart, often boasting about his battling prowess or how cool his Pokémon are. Barry does have a soft side, however, as seen after the wild Starly attack.
Barry is also one to take risks and quickly jump into action; it was this behavior that caused the player and himself to be attacked by wildStarly, setting off the chain reaction that would result in the player receiving their first Pokémon.
His father is Palmer, one of the Sinnoh Battle Frontier's five Frontier Brains. Barry wants to be like him, which is the reason he is so determined and trains hard. Barry is actually very much like his father characteristic-wise. Both he and Palmer are constantly in a hurry, running into people (including the player), and both seem to react the same way when they happen to run into people.
In addition to his hyperactive personality, it is also shown that in the beginning of Platinum, he gets distracted very easily. In the events of Platinum, Barry suggests going to visit Professor Rowan at his lab rather than go to the lake in search of a Pokémon of the same rarity as a red Gyarados (as happened in Diamond and Pearl). When Professor Rowan questions Lucas/Dawn and Barry on Route 201 about going into the tall grass without any Pokémon, deeming it reckless and wondering if such people were well-suited for Pokémon, Barry admits it was his idea and says that his friend should at least get a Pokémon if not him. After this, Professor Rowan gives them both Pokémon, much to Barry's excitement.