Catherine's girlish good looks and curvaceous figure belie an inherited strength that allows her to lift objects like pianos above her head with one hand. Catherine is shy by nature, but has stated that she prefers the company of men who are as muscular and strong as her elder brother. This may be an effect of how the Armstrong family values muscular strength and military backgrounds.
Catherine first appears in the Bonus Chapter: The Second Lieutenant Goes to Battle! (which is later adapted into Episode 37 of the 2003 anime series) in which she is formally introduced to Second Lieutenant Jean Havoc, who has been forced to end a budding relationship in East City upon being transferred to Central City. Despite Havoc's initial fears regarding Catherine's family bloodline, he finds himself attracted to her and the two appear to get along well. But when he asks Catherine out directly, she apologizes and says that she only dates muscular men who are like her brother, much to Havoc's shock and disappointment.
Catherine appears again in Chapter 83 when Olivier Mira Armstrong demands that Philip Gargantos Armstrong leave the Armstrong mansion to her and immediately vacate. He understands that his daughter is warning him of some impending danger and does so, leaving for Xing with Mrs. Armstrong and Catherine. As Olivier and Alex battle over the right to the mansion, Catherine kindly offers wishes of good luck to Alex, suggesting that she may prefer her brother to her eldest sister.
In Episode 38 of the 2009 series, Catherine is revealed to have thwarted Yoki's attempt to rob the Armstrong mansion when he accidentally startled her by running into the same room. She then threw her piano straight at him in a fit of terror.
- The name Catherine means "pure, clear" in Greek.
- Catherine's middle name Elle means "she" in French.