[Dark Mercury]

I don't have much of a preference personality-wise, I generally like main characters that get stuff done (as opposed to being reactive or passive) and secondary Riders that complement the main Rider in some way. Tendou/Kabuto is proactive, not to mention a total badass from the very beginning. Kagami/Gatack complements him by being relatable and giving us the struggle to become a Rider storyline we wouldn't have gotten otherwise. (I don't know if the story even needed more than two Riders. I prefer shows with multiple Riders, though, and the tertiary Riders were a huge part of why I enjoyed that season so much.)

On the other hand, Haruto/Wizard is also a badass from the beginning and he also fights to protect a girl that I couldn't bring myself to care about (sorry, Hiyori and Koyomi) but unlike Tendou, he has a forgettable personality. I guess he's nice? He only likes plain sugar donuts and that makes him quirky? (I also remember him being lazy, he rarely chased after the monsters when they ran away. ) Kosuke/Beast does complement him by being fun and outgoing, as well as having a more interesting storyline.

I also appreciate it when a main Rider is the perfect main character for the themes of their story, like Eiji/OOO and his relationship with desire.