Yang and Winry Rockbell have a good amount of common ground.
Ruby is the optimistic little kid who has big hopes and believes they can do anything, if they just BELIEVE IT! I'm not coming up with specific characters at the moment, but I know I've seen the type. (Albert Morcerf from Gankutsuou, though that's a pretty niche show.)
RE: Spike, I think he's waaaaaaay more easygoing than Ren is. He's confident, skilled, and incredibly lazy.