Ruki is the type of person who, in the beginning, sees Digimon as nothing more than mere computer data. She aspires for her partner, Renamon, to become the strongest Digimon ever. Eventually, she realizes that the two must be on equal partnership and friendship. She is a bit bitter and slightly antisocial thanks to her parents' divorce, but Takato, Jenrya and Renamon, manage to bring out the best in her, by showing her (among other things) that friendship is priceless.
Ruki's relationship with Renamon is unusual (due to Renamon being the most mature of rookie Digimon), they talk as equal than either a pet or a playful child. Ruki's proud, independent, grouchy and cynical personality is substantially different from most of the female characters in the series, pretty much a typical teenage rebel.
Her personality is due to her parents divorcing. She was closer to her dad, and her mom doesn't really understand her, always pushing the tomboyish Ruki to try and be a model. This left Ruki feeling isolated, and forced her to be defensive and rebellious. As a self defense mechanism mentally, she believes she "doesn't need anyone". Which is believed to account for the way she treats Renamon at first.
Deep down, Ruki is loving and caring: wishing to protect her friends as well as the city. Even though she puts on a "defensive act", being tomboyish is a genuine part of her personality.
Most anime shows featuring a team of adventurers have at least one girl on the team. These characters are often displayed as damsels in distress, but from Digimon Adventure to Digimon Xros Wars we see many ladies who can keep up with the boys on any day.