The second-hander
A person who lives through others' opinions rather than their own judgement.
Introduced in The Fountainhead, chiefly through Peter Keating: the second-hander's goals, values and sense of self are borrowed from other people. Roark's courtroom speech sets the creator against the second-hander.
Read it in Rand’s words
The Fountainhead (Roark's courtroom speech)