Antagonist doesn't mean just villain. It also refers to additional characters of great relevance but with the story not revolving around them like protagonists. As masaki said, this includes allies, sidekicks, anti-heroes and that stuff.

Gemstones are like ESPers but naturally born with. They are not necessarily more powerful than technologically given ESPers. ESPers are thoughfully categorized by Grey, but if you want to get an idea I like to think of them like the powers of the stands in JoJo, abilities that vary in power but that with some thinking results in a useful and crafty tool