Time for a history lesson. While Tommy is famously the first 6th Ranger of Power Rangers and Buria is technically the first of Super Sentai, there are some interesting stories leading up to Buria. Click "Read More" for the full tale.
Choudenshi Bioman - Magne Warrior
Hikari Sentai Maskman - X1 Mask
Sounds like what would expect from a 6th Ranger, so why isn't Ryo counted as the first? Sadly, because he was a one-off character. He lost his powers on the same episode he appeared and that entire arc was less than 20 minutes of screen time. No Zord or unique weapon. At least he eventually got a Ranger Key.
Shoto and Ryo have been dubbed the Prototype 6th Rangers. The real deal came in a familiar face.
Kyōryū Sentai Zyuranger - Dragon Ranger Burai
Despite all that, his short time on screen was amazing a beloved by fans. Such popularity caused the 6th Ranger to become a staple in the series.
The next few seasons would all have some sort of 6th or extra Ranger/Hero. They experimented with the formula, stronger with limitations, not adult or human, ect. It was a new concept, a Ranger that did not have to fit the status quo of the core and it allowed for new Super Robo Combinations.
Kyukyu Sentai GoGoFive - Beast-Demon Hunter Zeek
The rest is history. There were 15 incarnations of Super Sentai with 2 Prototypes before they tried out the concept of a 6th Ranger, and it stuck. It was coincidence that the first Power Rangers happened to use footage that included the first 6th Ranger, but Saban did attempt seasons before Zyurangers before finally getting greenlit. Thank you for reading, this has been Cendoo the Ghost Ranger, signing off.