Ever since the very first season all the way back in 1993, mysterious new additions to the Power Rangers have been par for the course, so much so that the Sixth Ranger has gone from exciting new territory to a deeply rooted trope of the franchise.
While fans have grown to expect the various Power Ranger teams to grow far beyond their primary colors, that doesn't mean they're incapable of being surprised, and no one has done that better than Boom! Studios. Not everyone who has been made a Power Ranger has left a particularly large impact on the team, but for those that did, they're still being felt to this very day.
Lord Drakkon
Lord Drakkon is probably the last person anyone would expect to become a Power Ranger, and if they didn't believe that he was part of the team, they would mostly be right. The alternate universe Tommy Oliver lorded over the Power Rangers for a long time after traveling the multiverse in his quest to murder every other version of himself in existence. Drakkon has committed some of the worst acts in Ranger history, and it took reshaping the universe itself to bring his reign of terror to an end. As evil as Drakkon is, he's also one of the only people alive with any working knowledge of Dark Specter and the Empyreals, which makes him valuable enough that the Omega Rangers were willing to kidnap him from their own former teammates. Even if he isn't officially part of the team, Drakkon is still right there alongside the Omegas in their search for answers whether he wants to be or not, and that's close enough to include him on the list.
Britt and Serge
When Kimberly left the Mighty Morphin Power Rangers to pursue gymnastics full time, she journeyed to Europe to train and compete. During the events of 2016's Mighty Morphin Power Rangers: Pink by Brendan Fletcher, Kelly Thompson, and Daniele di Nicuolo, she discovered that a small town had been taken hostage by Goldar. Still imbued with the power of the Sword of Light, Kimberly formed the All-New Power Rangers with some help from local siblings Britt and Serge acting as the Red and Blue Rangers respectively. The two had watched their town fall victim to Goldar's plot, and both welcomed the chance to fight back with open arms.
Bulk and Skull
Eugene Skullovitch and Farkas Bulkmeier have been the slapstick comedic relief of the Mighty Morphin Power Rangers since its debut, and while the comics have given them a bit more nuance than the original series ever did, they haven't changed all that much from their initial incarnation. They have, however, had the opportunity to take on the role of Power Rangers for themselves, even if they don't remember it. When one of Rita Repulsa's monsters was successful in devouring the Mighty Morphin team, Zordon temporarily deputized Bulk and Skull through two "extremely rare" Power Coins. Their time as the Purple and Orange Rangers might not have been long lived, but it was a brilliant moment of fan-service that will never be forgotten.
1969 Rangers
In 1969, the dreaded Psycho Green was awoken from his hibernation on the moon by a manned lunar lander. The threat of the former Supersonic Ranger was too great to ignore, and as such Zordon called on five young adults to act as the very first iteration of the Mighty Morphin Power Rangers. The team, led by Grace Sterling as the Red Ranger, traveled to the moon to deal with Psycho Green with high hopes. Unfortunately, they were wildly unprepared for what came next, and three of them lost their lives in battle. Grace and Terona Washington, the team's Yellow Ranger, returned to Earth with nothing but scorn for Zordon, who stripped them of their powers now that the world was safe again.
Kiya
After the universe was remade to save it from Lord Drakkon's influence, the alien Kiya was imbued with electricity-based powers that sadly resulted in the deaths of her family. Kiya was chased from her village and eventually found herself among Jason, Zack, and Trini as an Omega Ranger. She was easily the strongest of the team, not to mention the fiercest fighter, but when her past traumas caught up with her Kiya quickly became their most dangerous enemy. After killing the Blue Emissary, Kiya attempted to take down the Power Rangers entirely, operating under the delusional belief that they were only setting up the next Drakkon to take control of the universe.
Matthew Cook
Matthew Cook was one of the Rangers' best friends in school, even if he wasn't made a part of the team himself. In fact, Matthew wasn't even aware that his friends were the Power Rangers for quite some time, even while dating Kimberly. After being kidnapped and tortured by Rita Repulsa, Matt put the pieces together and confronted his friends about what had happened. When they couldn't bring themselves to be honest with him about their alter-egos, Matthew turned his back on the group. They were all able to reconcile eventually, but things never went back to what they once were. It seems like that's all changing in ways the Rangers never expected, though, as Matthew has just revealed himself to be the mysterious and powerful Green Ranger that has been helping the team for months, and all under the tutelage of none other than Grace Sterling.
Teenage Mutant Ninja Turtles
2019's Mighty Morphin Power Rangers/Teenage Mutant Ninja Turtles by Ryan Parrott, Simone di Meo, and Alessio Zonno gave fans one of the most spectacular team-ups of all time. When Tommy goes missing in New York City, the others go to rescue him, only to discover that their teammate has infiltrated the Foot Clan. When Shredder comes into possession of the Green Ranger's power and takes away the Rangers' ability to morph, their Power Coins are entrusted to the Ninja Turtles and April O'Neil, and the two teams head off to save the day.