Claus

Claus is a character exclusive to the 2003 anime, who lives in a village that the Elric brothers visit at the start of the anime during Edward's 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 became more feminine, something that shocked both Ed and Al as they couldn't 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.