There are about a jillion crazy things about the show BATTLE FEVER J (the name being the most obvious), but "Diane Martin" is right up there among the craziest.
As background, the very first sentai, GORANGER, had a female member with the unusual for Japan name of "Peggy Matsuyama". She was portrayed by Risa Komaki, who I'm fairly certain was all Japanese. Still, it was an interesting touch.
In the second series, JAKQ, the female member was "Karen Mizuki", who was an FBI agent from America. She was portrayed by a mixed race actress with the wonderful name of Michi Love. And yes, that seems to be her real name, too.
So we come to BATTLE FEVER J, and it was apparently decided to cast an authentic American as the female member. Though she is of Asian ancestry, Diane Martin (credited as D. マーチン) seems pretty obviously American. She is presented not only as an FBI agent, but the daughter of the purely Caucasian "Mister Boss" (David Friedman, but not the filmmaker of the same name). It's actually a pretty cool idea, and she's spotlighted heavily in the first episode.
Unfortunately, one can only guess that she was deemed unsuitable for the role after production was underway. After the first few episodes, she is shuffled off to the side. There are more than a couple of episodes where Diane makes a token appearance in Battle Fever's HQ, has maybe one line, and then doesn't appear until the climax in costumed form (portrayed by another actress). She never gets a spotlight episode to herself. For a long stretch, Battle Fever J appears to consist of four guys and, oh yeah, this chick that shows up at the end.
It's worth noting that I think Diane Martin, playing a character NAMED "Diane Martin", is the only lead actor in a sentai series to ever be dubbed by another performer. Risa Komaki, the first sentai heroine, returned to dub Diane's lines and also perform in the Miss America costume during the first half of the series. Given the acting chops usually displayed in superhero shows, this speaks volumes about Martin's acting ability and/or command of Japanese.
(Though Komaki wasn't in the costume for the whole series, it's sort of a fun trivia note - especially since she WASN'T in the suit in GORANGER as far as I know.)
Finally, in episode 24, Martin was written out of the series and replaced by a second Miss America. This second Miss America, Naomi Hagi as "Maria Nagisa", was immediately far more involved in the action than Martin ever was. She interacted with her male teammates more than Martin had since the first handful of episodes. She got episodes spotlighting her. She was part of the gang so fast that you're left saying "Diane who?"
Looking back on the series today, BATTLE FEVER J shows a lot of signs of being a troubled production. Besides the Miss America fiasco, there was also the replacement of Kenji Ushio with Masashi Ishibashi in the role of "Header" after filming was already underway. This led to a confusing string of episodes where some footage couldn't be reshot and the two switched back and forth between the role.
It's amazing BATTLE FEVER J made it to the air at all. But in spite of all the insanity, its 1979 debut started the super sentai cycle in earnest, and the cycle has yet to leave the broadcast lineup. That's rather impressive by anyone's standards.