This is one of the rarely covered Sentai, mostly because only four of the episodes are subbed. As well, it's hard to find resources that fully describe it. Most copy them from wikipedia, which barely describes the main characters. So basically, here is my summary:
Okay, 3000 years ago, an evil space empire, the vader clan (sound familiar, anyone), attacked the Denji Star. An island of the star, Denjiland, landed on Earth. In present day (1980 Japan), the Vader clan is finally approaching and detected on Earth. The Denji Computer awakens the talking Denjidog, IC, to find five people to be Denjiman. He finds, one by one, five individuals who may or may not be decedents of the Denji people (spoiler alert: they are), and they become the Denjiman.
Ippei Akaki was the first recruited by IC. He is a karate teacher at the Athletic club, and is well-respected amongst his students. Very wise, and a strong leader. As well, very skilled in karate. Mainly your standard Sentai leader.
Wait a second, does this guy look familiar? You bet he does. Daigorou Oume is played by Kenji Oba, who was previously Battle Kenya in Battle Fever J. Looks like he got a promotion, because he's now the Second-in-Command, Denjiblue (he doesn't get promoted to leader until he was on Space Sheriff Gavan). He was a former circus performer (acrobat, motorcycle rider). He went on to teach yoga at the Athletic Club. Like his character on Battle Fever J, he's sweet, child-like, and fun-loving. He's a bit of a playboy, and loves anpan--a lot. Anpan is a sweet bun with red bean paste, by the way. He is usually seen snacking on it, and has stashes of his favorite treats hidden in the athletic club. He has a whole locker full of them. Seriously, dude needs an anpan-intervention. Also, Kenji Oba was Denji Blue's suit actor as well. All and all, a fun character, and fun to watch.
If you want the prototype for Billy, look no further, although I doubt the producers watched Denjiman. Like Billy, he was very smart, developed most of the new technology for the team, and was their go-to-guy for science. He was discovered by IC when he was with a group of scientists, investigating a Vader Clan attack. You could find him most of the time in the Denjiman's fortress, wearing his glasses and lab coat, analyzing the science used in the attacks. Now, most sources say that Jun taught calisthenics at the Athletic Club, however, he actually taught healthy cooking classes, and usually cooked for the team, and for the kids at the athletic club. In addition to being a scientist, he was a really good cook (although Daigorou preferred his anpan over Jun's dishes). Jun is very scientific-minded, and very rational and logical. Unlike Daigorou, he is very shy around girls, and in one episode, fell in love with the daughter of one of the grocers he visits, who was shy too, except when she was possessed by a Vader Monster. Due to the fact that he isn't athletically trained like the rest of the Denjiman, and is mostly brains, he gets super strength in his Denji man form (okay, not superstrength, but he uses more force). Because of that, his attacks are usually wrestling moves. All and all, for a science-based team, he was an important member, who they wouldn't be able to function without.
As Jun's team was analyzing a Vader attack, Tatsuya Midorikawa was investigating it. Tatsuya Midorikawa was a detective for the police before joining the team. He sought vengeance after his father, a detective by his side, was killed by the Vader clan, after investigating them. Tatsuya is really cool, professional, but has a lighter side too. He teaches boxing at the Athletic club. As well, he is an expert marksman. He also plays the guitar. That just adds into the coolness factor. Unlike most of the Denjiman (except Akira) he had a reason to fight the Vader clan. As well, he has police connections, and two female cops who have a crush on him, usually follow him around and help the Denjiman whenever they can. Honestly, watch the series, and you'll see how cool he is. His actor, Naoya Uchida, went on to become a voice-over actor, and has aged nicely.
Akira Momoi was the second person recruited by IC, after her tennis instructor was burned alive by a Vader attack. She initially didn't want to join the Denjiman because of her tennis career, and owed it to her coach to keep going, but that changed after being affected by a Vader Monster. She gave up her tennis career in order to fight. Like Tatsuya, a person close to her was murdered by the Vader Clan. She went on to teach swimming at the Athletic Club. She fulfilled the role of a Sentai female, usually falling in love, using disguise, changing clothes, etc. She aged nicely.
Wait, she looks familiar too. Machigo Soga (aka Rita and Bandora) played the evil Queen Hedrian of the Vader Clan. She was hated humans, beauty, etc. Her rule was taken over by the Omnipotent Demon King, and her true fate was unknown after the defeat of the Vader Clan, until she resurfaced with new upgrades in Sun Vulcan. Even though she was a total bitch to humans, she cared about her subjects and the Vader Clan.
General Hedrer was loyal to Queen Hedrian till the very end, when he enlarged himself (I think) to protect her and the Vader Clan. Her field commander and strongman, he was able to assume a civillian form on Earth.
Mirror and Keller. Mirror's the one in the gold, and Keller is the one in the Silver. Mirror was able to turn into a Mirror for her mistress, Queen Hedrian's, use, and Keller could turn into a shield. They were the henchwomen, and assistants to Queen Hedrian, and served as spies and feme fatales on Earth. They were able to assume civillian disguises on Earth. They were loyal to Queen Hedrian as well, and Queen Hedrian cared about them.
What's notable about the series: the plot was standard, the roles on the team were close to standard (mainly the leader and the chick), but this introduced the first transforming robot (the robot in Battle Fever J didn't transform, I think), and created the prototyple helmet designs still used today. The themes are technology, which reacted to the bubbling technology industry in Japan at that time, and loyalty. The team was loyal, IC was loyal and sacrificed himself, and even the Vader Clan was loyal to themselves.
So yeah, watch an episode or two. It isn't that hard to figure out what's going on.