Sayaka Yumi is the main female character of the original Mazinger Z series.
The daughter of Professor Yumi, the head of the Photonic Research Lab, Sayaka was given the giant robot Aphrodai A to pilot against the forces of Doctor Hell, only to be placed in second place after Koji shows up with the Mazinger robot instead. This is justified since the Aphrodite was much weaker than the Mazinger, but Sayaka still felt a rivalry with Koji, especially since his macho attitude rubed her the wrong way. Then again her own temper was terrible.
Sayaka represented the liberated, modern woman that was still a new thing back when the first Mazinger series was created. Koji often mocked her, saying she should "stay in the kitchen" which made her angry. Then again, both Koji and Sayaka cared a lot for each other, as can be seen in episodes were they got badly hurt; they would just not admit it openly. Sayaka was actually smarter than Koji, and even knew martial arts.
One episode has Sayaka going into a great depression after the Aphrodite robot is destroyed (she even hallucinated seeing the robot call to her.) However she got over it when a new robot, the Venus A, was made for her. The Venus was still not significantly better than the Aphrodite, however.
At the end of the series, Sayaka left Japan with Koji to study in the United Sates. She had cameos in other series later (such as in Great Mazinger) and has been included in most of the Mazinger remakes.