From Kamen Rider Agito: Kamen Rider Gills
Kamen Rider Gills, a very good example of a rider with great powers, but it comes at a price to his health and sanity. I would consider him under the category of the Joker, as his powers were unnatural and almost like dealing with an inner beast who's power was able to evolve and get stronger, but still at a cost to his human side. But eventually he was able to master his powers, and be able to control it.
From Kamen Rider Blade: Kamen Rider Chalice
Kamen Rider Chalice, the first confirmed Joker. A rider whose real form an Undead, the Black Joker. His transition from Rider, Human, and Undead was very interesting in the series. His story was a tragic one, trying to come to terms with Humanity, and not being able to reach that goal for himself. He had two Rider forms, the normal one, and then his King Form, Wild. His "Death" was tragic at the hands of Blade, as he and Blade were close to being friends.
From Kamen Rider Kabuto: Kamen Rider Dark Kabuto
Dark Kabuto was a worm mimic of Tendou Souji, and existed in an alternate dimension. As a dark mirror of Tendou, Dark Kabuto was very powerful, but his emotions were his weak point, as Kabuto was able to defeat him using his logic. Dark Kabuto was the Joker in this instance because he was the dark version of the main Rider, and knew everything Tendou did.
From Kamen Rider Den-O: Kamen Rider Den-O
Ryotaro Nogami, a young man with very bad luck, one day becomes the bearer of the Den-O belt. He is a Singularity Point, a human who can exist separate from time itself. When he allows one of his partner Imagin to possess him, he then becomes the Den-O form associated with that Imagin. Momotaros, Urataros, Kintaros, and Ryutaros are his Imagin partners. When all 4 Imagin possess him at once, he can become the Climax Form. When they cannot possess him at all, they came up with a final Liner Form for him. I see Ryotaro as a Joker, being able to be a nexus outside of time and allowing him so many powers from different Imagin.
From Kamen Rider Double (W): Kamen Rider Joker and Kamen Rider Skull
Kamen Rider Joker and Kamen Rider Skull. Users of the Lost Driver, both their singular Gaia Memory henshins hold unknown powers. As of recent, the powers for Kamen Rider Joker have been seen as basic Rider attacks, but with a lot of power behind them. Kamen Rider Skull rumors floating around now from the new Kamen Rider x Kamen Rider OOO movie reveals that Skulls first henshin is known as Crystal Skull, with his full powers yet to be revealed. This to me shows these two really do fit the category of Joker.
