Claus is a character exclusive to the Fullmetal Alchemist 2003 anime series, who lives in a village that the Elric brothers visit at the start of the anime during their flashback.
When they first meet her, she is rude and acts like a tomboy who wishes to avenge her sister's death after she was killed by one of the Karin zombies. She briefly tries to persuade Ed to help her, but Ed rejects her, saying he doesn't want her to pursue revenge. She antagonizes him again, and tries to attack one of the Karin golems, only to be saved from it by the village alchemist, Majhal. In fact, Claus's older sister had been one of Majhal's first victims in his deluded hope to bind the soul of Karin to a mannequin made to look like the young flower seller.
Claus puts herself in danger when she arrives at Majhal's home to thank him for saving her, and she sees that Majhal is attacking Ed and Al. Both to eliminate her as a threat and in one last attempt to bind Karin's soul to a young girl, Majhal attempts to use Claus and initiates the transmutation. However, after being saved by the Elrics from Majhal's evil scheme to bring back the real Karin, Claus warms up to the Elric brothers and becomes more amiable.
When she heard that the two brothers were going to leave the village to head their next destination, she rushed to see them because she wanted to apologize for the way she acted from when they first met. She apologized in the same way she used to be before she lost her sister, and both Ed and Al were shocked as they could barely recognize her in a dress with her hair down.
While some may argue that Ed and Al believed Claus was a boy, Ed had twice told her that she should dress like a girl, not a paper boy, which results in her claiming to be taller than Ed, and so on.