The Power Rangers included five very different personalities, but the one that changed the most over the course of the show belonged to the Blue Ranger Billy Cranston.
Billy started the show with a very visible lack of confidence. His intelligence was his most powerful weapon, but even before he became a Power Ranger he struggled with a variety of insecurities. That intelligence would come in handy though, especially in the early days of the Power Rangers, before they truly had a chance to grow comfortable with their abilities.
If you started watching Billy in season one, you would most assuredly notice a huge difference in the character by seasons end, which only continued into the show's later seasons and beyond. Billy had arguably the biggest arc out of any of the Rangers, growing with the audience to a degree as he found his role on the team and increased his skill set.
In the beginning, Billy couldn't fight outside of his armor, bu8t he would later train under Jason to increase his martial arts prowess (with a few handy tips from Trini's uncle as well). He would also become more confident outside of his armor, and eventually would ditch the overalls only look he sported early on.
Billy has been played by two actors (officially that is). The Billy seen in the television series is played by David Yost, while the version that appears in the recent live-action reboot is played by RJ Cyler. In the original version, Billy would command the power of the Triceratops Dinozord, but would later go on to command the Unicorn Thunderzord when those were destroyed. Eventually, those would be destroyed as well (thanks, Rito), but he would get another upgrade to the Wolf Ninjazord and could even control the Blue Shogunzord.
He would also battle with the Power Lance, which could be used as a lance or pulled apart into twin lances.
Mighty Morphin Era
As mentioned before, Billy started off very insecure in his fighting prowess, but he did know he could help with his brain.
With Trini often at his side, he would create several devices that would get the Power Rangers out of jams, but he also created their communicators and along with Alpha figured out how to use them as teleportation devices to the command center.
He would later keep Alpha 5 from exploding by disarming his self-destruct sequence, and would also create the RADBUG, which helped the Rangers get to the command center when teleportation was cut off.
Billy was one of the few Rangers who stayed with the team throughout the entire Mighty Morphin era. He was there (and discovered the process early) when Tommy became the White Ranger, as well as after Jason, Trini, and Zack left the team, replaced by Aisha, Rocky, and Adam. He was one of the first to meet the new recruits actually but also was responsible for giving away the Rangers' secret identities to them.
Hey, can't win them all right?
Billy even had an ancestor in the 1880's that Kimberly recruited to be the first Blue Ranger in the Wild West, all thanks to an unplanned time warp.
Mighty Morphin Alien Rangers
Rita Repulsa's father Master Vile managed to create a spell that turned the Power Rangers back into children, but Billy's intellect managed to come in handy once again.
He would create a machine that used the Power Coins to restore the Rangers, and would use it on himself. While he restored himself to the proper form, Goldar took the machine before he could restore everyone, and Zedd and Rita made sure that wouldn't happen again by destroying the other Power Coins.
Billy couldn't morph anymore, so he acted as guide and advisor to the Alien Rangers.
Billy let Tanya Sloan take his place in the field during the Zeo years, as he moved to a more consultant and guide role.
That is until Trey of Triforia needed someone to take on his Gold Ranger abilities while he recovered. Unfortunately, he had absorbed too much negative proton energy from the initial command center explosion earlier on, and couldn't take them on. They were instead given to Jason Scott, who Tommy had recruited back into service.
Billy's earlier experience with his aging machine (during Alien Rangers) was also discovered to have a side effect. He had started to age rapidly, becoming older by the day. He would travel to Aquitar for a cure and managed to stop the aging thanks to the Eternal Falls. Even after his age was restored, he would stay on Aquitar with the one he had fallen in love with Cestria.
Billy was brought back once more during the "Legendary Battle" episode of Super Megaforce, alongside the rest of the original Mighty Morphin Power Rangers.
The Super Megaforce Rangers fought their hardest against Emperor Mavro, but were on their last legs and without Zords as he made his final assault. A Legendary assembly of Rangers would arrive to help them fight off the final invasion, including the original Blue Ranger.
They ultimately pushed back the XBorg and Bruiser army and would return to the past as quickly as they appeared.
Unofficial Fan Film
In Adi Shankar and Joseph Kahn's vision of the Power Rangers, the Machine Empire ultimately beat the Power Rangers by destroying their Megazord.
Earth would enter into a treaty with them after the Rangers fell, which then meant the Rangers were disbanded. During Rocky's interrogation of Kimberly, it is revealed that Billy became a successful weapons manufacturer, but it is shown that he also committed suicide.
That ends up being a ruse though, as Billy was truly killed by Rita Repulsa, disguised as Kimberly.
Power Rangers Reboot
Billy experienced some changes in the live-action reboot, but many of the hallmarks are the same.
In this version, Billy is still vastly intelligent and a bit nerdy, though here the character reveals to Jason that he is also autistic. He's much more talkative in the reboot though and is used in more comedic moments than the original was.
Billy is also the very first one to morph into a Ranger and ends up being the gel that keeps the team together. While the original was an important part of the team's makeup, the reboot's Billy is easily the heart and soul of the team.