In "The Fall of the House of Usher," what does Roderick symbolize?

When considering symbolism in Gothic Literature
(especially Edgar Allan Poe) I think it is safe to say the following ideas are probably
represented somewhere:

- darkness (physical, mental,
emotional, moral)
- evil
-
secrecy

Consider Roderick's
characteristics:

- hypochondriac and disorder of heightened
senses
- artistic
- keeps to himself
- generally kind but
clearly hiding something

I think it would be safe to argue
that he is symbolic of a family secret, an evil.  Most probably interpret it as incest. 
He is the last of the Usher line, which means the secret dies with
him.