From Bulbapedia, the community-driven Pokémon encyclopedia.
Bonnie (Japanese: ユリーカ Eureka) is a traveling companion of Ash who debuted in Kalos, Where Dreams and Adventures Begin! along with her older brother, Clemont.
Clemont and Bonnie were both standing outside the Prism Tower when they watched Ash go inside. Minutes later he was ejected out of it for not having any badges from Kalos when the requirement is at least four. Clemont used his backpack which converted into a giant airbag that provided a soft place for Ash and his Pikachu to land. Pikachu got catapulted upwards and Bonnie was able to catch him. As soon as he was in her arms she began to hug him tightly which caused him to use Thunderbolt on her and the two boys. Clemont scolded her for it and made her apologize to Ash and Pikachu for what she did. She did and explained that she was too young to have her own Pokémon and always was really excited when she got to meet one. Ash related to her on that saying he was the same way.
Later on she refereed the battle between Clemont and Ash and was greatly impressed by the both of them. The battle was then interrupted by Team Rocket, who tried to attack Pikachu but the assualt was blocked by a mysterious Froakie. Seeing how hurt it was, Clemont suggested they take it to Professor Sycamore's laboratory and Bonnie led the way.
In Lumiose City Pursuit!, Bonnie was able to pet a Garchomp that Sycamore was researching while they waited for Froakie to be healed. Out in the yard, she got to meet even more Pokémon before Team Rocket infiltrated the lab and put a device on Garchomp so they could control it. It ultimately climbed up Prism Tower and she tagged along with Ash and Clemont and watched her brother use one of his inventions to break in. She and Clemont couldn't go on any further after part of the tower collapsed, blocking off the only entrace, leaving Ash and Pikachu to go it alone.
The next day, Bonnie and Clemont decided to accompany Ash on his journey through Kalos.
Bonnie is a young and cheerful girl who is always excited to meet new Pokémon. At times she gets too invasive with Pokémon, similar to Iris and Bianca, causing them to attack her. She is very proud of her brother's accomplishments as an inventor but sometimes criticizes him.