In the Mario series, namely the spinoffs, there are many playable characters to choose from. On the friendly side, the playable characters tend to be the unique ones like Mario or Peach, or they are famous species like Toad and Yoshi. On the villain side, they also have unique characters like Bowser. However, one big difference is that a lot of enemies got to be playable. Whether it's the famous ones like Koopa Troopa or Lakitu, or the ones that aren't as much in the spotlight like Ninji, or surprise picks like Chargin' Chuck, a great variety of them got to be playable.
The screenshot above is but a number of friendly species. Some got the occasional playable appearance, but some didn't. For example, there's Dolphin, which got one playable appearance in Dr. Mario World, or perhaps the Piantas and Nokis got to play ball in the Mario Baseball games. And then some haven't got to be playable yet, most noticeably the Penguins. Pretty much only Toad, Yoshi, and to a lesser extent Birdo got enough playable appearances to be a regular, whereas the enemy species have a much bigger variety.
I suspect that there is not as much of an appeal in playing as the friendly guys, whereas there is more to like from the enemies. After all, they have varied designs and can be cute, like the simple Boo and the jolly Koopa Troopa. But perhaps there is a downside to it: some fans criticise adding enemies as playable to pad out the roster, so what's to say that the criticism doesn't apply to the the "friendlies"?
If you could pick a friendly species to be more frequently playable, regardless of if they've had been playable before, who would you pick? We are going for the lesser-known ones here. Of course, the selection need not be limited to what I posted in the screenshot.
Thank you for reading.