Nakane is Negi Springfield's older cousin. She is the one who looked after him and raised him before he left on his quest given to him upon his graduation of the Magic Academy.
Negi's cousin, although their relationship is more like that of brother and sister (and he refers to her as such), since they grew up together (and even slept in the same bed) after her father took the orphaned Negi in. Her gentle personality is largely based on interpretations through disjointed flashbacks, so it is hard to identify precisely what she is like. What has been indicated however is that she took on more of a motherly role in his life. As a side note, Asuna and Nekane feel similarly for Negi. Six years ago, when demons attacked the village where she and Negi used to live in, she was almost turned to stone while trying to protect Negi from a petrification spell, but was saved by the Thousand Master just in time. Her legs were turned to stone, however, and broke off. A later shot of her in Volume 10 reveals that her legs have been restored, although it is not clear if they were magically restored, or if she wears prostheses.
Although initially appearing as a background character, Nekane does gain a greater role in the second anime. As the Star Crystal crisis slowly grows, Nekane chooses to assist her cousin by disguising as the Black Rose Baron, an enigmatic character initially seen not working for either Negi or for the one controlling the crystal. Eventually, Nekane appears as herself to assist Negi and his class in the running of the day-to-day activities, but Negi eventually discovers her secret. However, he is forced to keep Nekane's other identity a secret to protect her and his students.