The mightiest pirate alive, Kaido is one of the most important characters in One Piece. Set up to be the main antagonist of the Wano Country arc, he has paramount importance to the overarching plot and has fulfilled quite a few roles in the story. Throughout the course of the arc, Kaido has been established to be the greatest obstacle for Luffy to accomplish and, at the same time, has had connections to the likes of Whitebeard, Roger, great history with Oden, and even ties to the legendary Rocks. At the same time, Kaido is heavily tied to the Joy Boy lore in the story, which just goes to show how multifaceted his role has been so far.
While it goes without saying there is a lot of depth to Kaido's character, the most important role that he has played in One Piece so far is that of being the greatest obstacle for Luffy to overcome and oversee his transformation into Joy Boy, and he achieved that by becoming the strongest. Kaido's strength is absolute and he's the strongest there lives in the world right now. On multiple occasions, he's been called the mightiest pirate alive and on others, he's openly been referred to as the strongest creature of all living things. Kaido's strength is what defines him the most and much of that comes down to his incredible Devil Fruit, the Mythical Zoan type Uo Uo no Mi, Model: Seiryu, or the Fish-Fish Fruit, Model: Azure Dragon.
Kaido's Devil Fruit Forms
Kaido being a dragon has been alluded to as early as the Punk Hazard arc through the Samurai of Wano, such as Kin'emon. In the Wano Country arc, his powers were properly revealed for the very first time shortly before his clash with Luffy. As a dragon, Oda established Kaido as a god who ruled over the land of Wano. In fact, Kaido was worshipped and paid tribute to.
His dragon form's might was godly indeed, as seen when he clashed with Luffy over Kuri in Wano and absolutely crushed him in battle. As with all Zoans, the dragon form is Kaido's beast form, and it gives him an incredibly tough skin, along with the ability to produce fire blasts, wind slashes, hurricanes, and even lightning roars. While his dragon form is certainly powerful, it's not his strongest.
Kaido's hybrid form was revealed to the fans during the war atop the roof of Onigashima when he faced the Worst Generation. His hybrid form gave him the power of his dragon-self condensed into a humanoid body, making him even stronger. Kaido's hybrid form is exceptionally powerful and took even the most standard attacks of his to the next level, making them hit harder and faster. Even an attack as strong as Zoro's Moja no Tawamure wasn't enough to knock him down to his knees.
At the same time, Kaido's hybrid form has further forms which he can access throughout combat, which just goes to show how incredible his control over his Devil Fruit really is. Most of his forms can be accessed while drunk, using a technique called Shuron Hakke. So far, Kaido has shown forms such as Thieving Drunk, where he steals the techniques of others and moves incredibly fast, and Homicidal Drunk, where he gets buffer and hits with more killing intent.
Other drunk modes that he's shown over the course of the fight include Laughing Drunk, Crying Drunk, Depressed Drunk, Angry Drunk, and Flirty Drunk, all of which greatly improve his adaptability in battle.
Kaido's Flame Dragon Torch
While all of Kaido's powers are special, his ultimate ability that he pulled off against Luffy was the Flame Dragon Torch or Kaen Daiko. When in this form, Kaido produced a giant flame dragon avatar that engulfed his entire body. The temperature of the flames was such that they resembled magma in appearance and, at the same time, could vaporize anything in their path without much trouble at all. Kaido could even vaporize Luffy's Gear 5 fist if he wanted to in this form, however, the latter made use of Ryuo to keep his distance and attack him from afar. Flame Dragon Torch is, without a doubt, Kaido at his very strongest. Not even someone as strong as a Yonko can get near him in this state, as seen when Luffy has to let go of him due to extreme temperatures.
When in his Flame Dragon Torch form, Kaido can easily obliterate an entire island simply by passing over it since he's bigger than Onigashima in size and is surrounded by high-temperature flames capable of vaporizing anything on contact. Although Kaido was eventually overpowered by Luffy, he is, undoubtedly, one of the best Devil Fruit users in the story and his majestic control over his many Devil Fruit forms backs that up even further.