Miscellaneous ideas topic.
I was unsure where to put this, but...Chisame isn't as cynical as she wants everyone to think. her cynicism is mostly a mask so no one will be able to figure out the truth about her.
You see, roughly 5 years ago, she heard a telepathic call in a small forest, and ran to find a ferret in the woods. She got it to a vet, but then it called for help in the middle of the night. She was given a magical staff, changed into a magical girl, and had a lot of fun and exciting times collecting Jewel Seeds. She made good friends, knew real, live aliens! and was going to join a peacekeeping group in another dimension!
But all good things must come to an end. Her parents aren't the Takamachis, and they don't have as much experience in crazy stuff happening, so while Chisame still keeps in contact with her friends, she feels left behind because they are all becoming important people in the TSAB, and by the time she becomes an adult and is able to leave her parents, she probably won't be able to see them even if she does join.
The fight with the Wolkenritter was the kicker, though. She had her Linker Core drained, and it put her in the hospital. That finally told her parents that she was getting into something dangerous, and they tried to convince her to stop. Unfortunately, the young girl's will wasn't enough to overcome her parents' at that age, and retired when the Book of Darkness case was finished. Sort of.
Chisame isn't willing to let this go, however. She occasionally fights monsters and minor rogue mages outside of classtime as a vigilante. Her persona is none other than the Magical Girl Chiu!
Mahou Shoujo Chiu is a small celebrity among the Earth-born mages and the Mahounet(or whatever it's called) She knows about and is able to access it because during one of her escapades moonlighting as a magical girl she came across someone from the Kansai Magic Association who gave her some information about the magical world and told her that she could keep on doing what she was doing as long as she didn't reveal magic.
When Negi stops that eraser in the first day of class with an obvious usage of magic, she decides to watch him carefully. There are several times that her new teacher gets into crazy situations that can only be the result of magic, but she holds her tongue.
However, she reveals herself when Eva attacks, using minor disguise magic and introducing herself as "Magical Pretty Girl Chiu," and does her best "stop in the name of love and justice" speech, before attacking. Chiu manages to drive off Evangeline, before giving Negi and Asuna her card (she has several she keeps whenever she goes out as a Magical Girl.
Negi is starstruck, Asuna finds it ridiculous that there is a genuine magical girl on campus, and a new story is born.