Took a long break from this but Here's the Dark Rider for my newest Fan Rider: Kamen Rider Lagos
Lagos' theme is based on the concept of Lucky Trinkets and charms. The series has an overall theme of Luck, so the idea with Lagos would be that he can steal luck for himself which is a big threat to the Main Rider Vulpine, whose entire power set is based off of Luck.
His main form is based on Rabbit Foots which are considered a lucky charm. I haven't drawn the other forms but they'd basically change out the right arm on the base suit to be themed after the lucky charm he activated with his Boost cards, offering different effects. He also has access to Legend Cards based on past Dark/Evil Riders allowing him to borrow their abilities. His final form I haven't designed yet, but it will have a similar vibe to Desast from Saber.
His Summon Beast Partner is Lagohare, a pun on Lagomorph + Hare + Nightmare. He is a nefarious Summon Beast who can leech power from others. Lagohare and Kitzu (Vulpine's Partner) seemingly have a history and are enemies. Lagohare's finish card is known as "Lagos Execution" which sends all of the User's power into one leg which unleashes a gigantic burst of energy through a kick.
Lagos is a member of GAMBIT who previously was known as Espion Rott. Using the Drawdriver model: Rott, he becomes Kamen Rider Lagos. Unlike Jinx (3rd Rider) and Thrash (4th Rider) he remains loyal to GAMBIT and their operations. He opposes Vulpine in both ability and personality, which makes him Vulpine's perfect rival.