Luna first appeared in Megaman Star Force as the class president of her 5th Grade Class in Echo Elementary. Therefore, she felt like she needed to push Geo Stelar (who later became Megaman) to go back to school, oblivious to the fact that Geo is still mourning for the loss of his father, and that he felt very anti-social at the time. It wasn't until later that Geo would relent and join them. She was doing it so she could have more fame in school or in Echo Ridge, rather than caring for Geo.
She first met Megaman when he defeated Taurus Fire, who took over her friend Bud Bison. From then on, she was in the thick of most of Megaman's fights with other F-Mians, and she started to grow a fond liking of the hero (In fact, she has a crush/die-hard attitude for Megaman in the cartoon). But after the series of escapades, her demanding socialite parents decided to move her to another school. Nothing went wrong until she ran into her parents while she was tailing Sonia Strumm and Geo who were going shopping. Things exploded and Luna really wanted to get at them. That's when Ophiuca came to her, and asked her to let Ophica in to give her power. Together, they formed Ophiuchus Queen. They attacked the parents and even stunned Sonia, but Megaman stopped her. In the end, Luna reconciled with her parents and she won't be moving to another school.
Later on, after another save from Megaman, she knew that Megaman was Geo, and thus she (with her friends) and Geo grew closer. She also shared a Brother-Band with Geo.
Personality: Luna could be mean, demanding, and a "my way or the high way" type of girl. This is probably because she is the new class president and the spotlight is constantly on her and she doesn't want to mess up on the stage. It could also be that she was raised in a luxurious setting with strict, demanding parents. Speaking of demanding, she is exactly that to her associates. She constantly tells them that they need to sharpen up and that they need to fill her high expectations. She is also fame-hungry and would do anything to be popular and among the elites in her school. However, that tough exterior belies her lonely nature. Like Bud (and maybe Zach) she doesn't want to be on her own, and she is in desperate need of support or even a shoulder to cry on.
As already said, she is a fanatic Megaman fan and knows that Geo is Megaman. She admits her feelings to Geo and Megaman, and there might possibly be competition for Geo between her and Sonia Strumm.