Basically, everything is copyrighted, but in order to violate a copyright you’d have to design the same character and tell the same story with him/her. (Long story short)
The problem with a character who looks like superman isn’t a copyright issue it’s (for instance) that the superman logo is a trademark and that “this is a job for superman” has its own legal status.
But create a big strong invulnerable flying guy dressed in red white and blue and call him Titan (like Dark Horse did in the 90s, cf Will to Power), and this is perfectly kosher. 🙂