From Bulbapedia, the community-driven Pokémon encyclopedia.
Bart (Japanese: バール Bar) is a character of the day who appeared in Unfair-Weather Friends.
Bart worked at the Weather Institute, where he was in the middle of training Millie, a new employee. He was also in charge of protecting data on ancient Pokémon. He had all of the information on a disc that he kept with him at all times.
When Team Aqua attacked the Institute, Bart gave the data to his Castform and told it to give the data to Millie. Along with Ash and his friends, Millie kept the data out of Team Aqua's hands.
After Bart had been captured by Team Aqua, he refused to give up the information about the ancient Pokémon. He told Millie not to either. But then, she revealed that she wasn't Millie. It was actually Brodie, a Team Magma agent in disguise. He flew away on his rocket pack with the data on ancient Pokémon.
Bart was freed by Ash and the others. Moments later, another person named Millie showed up at the Weather Institute. This time however, she was the real Millie. Bart showed her to a room where there were hundreds of books filled with data on ancient Pokémon. He told her to put all the information into the computers, which she gladly did.