When she was growing up, Nanami had the luxury of a wealthy environment. She spent her childhood as a typical rich girl, wearing beautifully impeccable dresses, enjoying the lavish parties thrown by her family. The details concerning Nanami's parents are sketchy at best; we only see them in flashbacks, and the one we are shown is one of her parents scolding her for bringing a rather dirty kitten to Touga's birthday party. In truth, Nanami and Touga, though related by blood, were actually adopted by the Kiryuu family. During her childhood, Nanami idolized Touga; he was her protector, her shining knight. Touga was the only real influence in her life, and it is because of this she loves him dearly.

Growing up, Nanami and Touga were very close, so he became the biggest influence in her life. She spent as much time as she could with him; they had slept in the same bed and taken baths together. But time was her greatest enemy. As the years went by, Nanami realized that her relationship with Touga was changing. They were maturing -- Touga, becoming a young man, and Nanami becoming a young woman. And suddenly, Touga wasn't interested in being as close to his younger sister. Nanami's realization of this becomes painfully apparent as time goes by, as is her frustration at having to be separated from her brother. Since she was helpless against the march of time, she instead devoted her time to keeping others away from Touga. Whenever something threatened her monopoly on Touga's love, she moved to methodically eliminate the threat.

As mentioned above, Episode 10 reveals a flashback of Nanami's childhood, concerning a scruffy little kitten she had given to Touga for his birthday. Although their parents protested, Touga adored the kitten and often played with it. Nanami was overjoyed that Touga loved his present -- but in time, Touga began to pay more attention to the kitten than he did to his sister. In a fit of jealousy, she put the kitten in a box and threw it in the river, telling herself that it was the kitten's fault, for coming between herself and Touga. Though she was obviously torn about killing the kitten, even at such a young age, Nanami demonstrated that she would be willing to do anything to keep Touga to herself.