Should I introduce a new hero, or a new villain?

I already have seven heroes in my Neutron Girl book. Should I introduce a new villain or a new hero?

The character I had in mind is a hero who is like movie Sebastian Shaw. It's a female who absorbs most forms of energy to convert into super-human feats. However, I don't know whether I have room for yet another hero in my story. I seem to be neglecting some hero characters already, and if written as a villain, this character would require something special to defeat from all of the heroes working together.
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Should I introduce a new hero, or a new villain?
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