From the Super Mario Wiki

It has been requested that this article be rewritten. Reason: the article literally states that it speculates about the character (tagged on 21:39, 6 October 2016 (EDT)).

Kaoru engaged in Super Mario Bros. 3.

Kaoru is a fictional Real World young girl who has made her only appearance in the Super Mario no Shōbōtai anime. As said appearance remains untranslated, her personality and mannerisms are guessed for the sake of this entry.

Kaoru appears to be a fan of games as shown by her dedication to her run of Super Mario Bros. 3 in the anime's opening scene. She also seems to disagree with her younger brother, Tatsuya as evidenced by her slightly rude tone of voice while playing. Since she once took her mother's sparklers without asking, she may be somewhat mischievous.

Later on in the short, her relationship with her father differs from the kinder tone she has with her mother. Kaoru also seems to be used to her father's drunken escapades and disapproves of them, denying him a kiss as well as sassing him when asked to pour a bath for him. Due to Mario's advice earlier on in the film, she also is one to spring to action when earthquakes hit, informing her mother about the running gas stove.