The other result in this direction is that if $\rho$ is irreducible, then $|\chi(g)| = n$ if and only if g is in the center of G/Ker. The proof is to start wiht what Qiaochu said, namely that by the triangle inequality and the fact that the eigenvalues are roots of unity you get that $|\chi(g)|=n$ if and only if g is a scalar matrix. Thus they commute with everything in End(V), and hence lie in the center of G/Ker. Conversely, use irreducibility to show that commuting with everything in G/Ker implies that you commute with everything in End(V).

When I took representation theory with Lenstra this argument was very memorable. He had started out in complete generality (arbitrary fields etc.) and as the course went on we needed more and more assumptions (algebraically closed, characteristic prime to the size of the group, etc.). When he got to this argument he said "Now this is the only time that we need to assume that the field is the complex numbers. This argument doesn't work over an arbitrary algebraically closed field of characteristic zero. (Although it's still true for such fields by model theoretic arguments.)"

A good related theorem to try to prove when you're thinking about the question you asked is that a representation is faithful if and only if every representation appears inside one of its tensor powers.