If someone is "two-faced," you can't trust them. They might say one thing and do another. They might be nice to you when you talk to them, but then spread rumors about you to your friends. Given that, the opposites would be honest or truthful, but also loyal and trustworthy.

"Two-faced" means deceitful, so the antonym would be honest or truthful.

The mean is algebraic or geometrical; the algebraic mean is the sum of the terms, divided by the number of terms, assuming there is at least one term. The geometrical mean substitutes sum with product and division with N-th root extraction were N is again the number of terms. An interesting related mean is the algebraic-geometric mean, that led Gauss to make interesting discoveries about finite integrals of the elliptic kind, which are found in Physics, and approximations of Greek PI (3.14159 ...) Does this make any meaning to you ?