For as long as she can remember, the centre of Nanami's life has always been her brother, Touga. Her brother is her ideal man, her idol, and the person whom she respects and loves above all others. All of these are facts she is quite willing to expound upon at length to anyone who will listen and a good portion of those who will not. And for the most part, Touga has proved willing to play the protective older brother to her doting little sister. And if some people might look askance at the closeness of their relationship, Nanami only smiles, knowing they're jealous of the special, unique relationship she has with Ohtori Academy's most envied and admired student. As Touga has been incapacitated for some time following his duels with Utena, Nanami has stepped up to take over his duties at Student Council President, which none of the other members have deigned to dispute.
Nanami is acutely aware of her social status as both Touga's sister and a wealthy member of high society. She expects obedience and adulation from lesser students, and she works hard to reinforce her image as a wealthy and beautiful social dynamo. Several students follow her around, trying to bask in her reflected glory, and Nanami magnanimously accepts this... and puts them to work. When her desires are thwarted, Nanami can become quite childish in her attempts to gain retribution, which she usually tries to accomplish through the proxy of her followers, leaving her own hands clean. Her temper tantrums are really more petty than actually cruel, but Nanami can be quite thoughtless as to the effect her actions have on others. The one area where Nanami accepts no compromises or superiors is her relationship with her brother. She is very protective of him, and while she grudgingly accepts that Touga will date women whether she wishes it or not, she will quickly become hostile to any girl who seems to have an actual hold on his heart. Still, to those who do not get off on a bad foot with her, Nanami is generally pleasant enough. Occasionally she's even quite kind, though it's a tendancy she keeps well-hidden.
Nanami, despite her own self-image, is extremely young, gullible and above all naive. She knows next to nothing about many adult subjects, and less than nothing about sex. Her body's entry into maturity has preceded her mind's, and her true feelings for her brother are sometimes murky and confusing to her... a condition Touga does absolutely nothing to rectify. Touga has also neglected to inform his sister that the two of them were actually adopted, as Nanami was too young to remember when it happened.
Nanami, while aware of the duels and some of the other strange events at Ohtori, pays less attention to them than the other Duellists. Indeed, she was a late entry, almost considered to be substituting for her brother, who was injured at the time. While she claims to have studied her brother's swordplay, and is certainly athletic, her skill cannot be considered that great, as she was easily defeated by Utena Tenjou.