Yin Yang to Copy ☯

The yin yang (☯) represents the duality of Chinese Taoism - two opposing forces that complement each other. In a nick, it easily carries that idea of balance, light/dark or good/evil, and it's one of the most recognizable visual marks that needs no explanation.

For a less overused look, it's worth trying the Tibetan variations ࿊, ࿋ and ࿌, which keep the circular structure with extra elements. The half-circles ◐, ◑, ◒ and ◓ work well in a duo: each player uses one half in their nick to complete the idea of a "full pair".