Female

With all the women on Bo's side, any of them would be perfect for this. The only question now was which one? The valkyrie, Tamsin would be good, especially with her power to cast doubt, and the fact she was still dark enough that no one may suspect she was acting in her true nature. There were also the humans, Lauren and Kenzi, with Lauren's knowledge of science and fae's would be helpful, and Kenzi's ties to Bo would let her get closer. Although making a doppleganger of Bo herself may allow for him to take out the main problem at it's source. Even then, there were always other options.